State the Law of Conservation of Mass .

The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction. The total mass of the reactants always equals the total mass of the products , which is why all chemical equations must be balanced .

A student reacts hydrochloric acid with calcium carbonate in an open flask. The mass of the flask decreases. Explain why.

The reaction produces carbon dioxide gas . In an open flask, this gas escapes into the atmosphere , so its mass is no longer recorded by the balance. Mass is still conserved overall. (Important: do not say evaporation. The gas is produced by a chemical reaction, not a state change.)

If a flask is closed during a reaction, the total mass stays ________ . If a flask is open and a gaseous product forms, the mass will ________ because gas ________ into the atmosphere.

If a flask is closed during a reaction, the total mass stays constant . If a flask is open and a gaseous product forms, the mass will decrease because gas escapes into the atmosphere.

True or False? When balancing a chemical equation, you can change the small subscript numbers in a formula to make the equation balance.

False. You must only change the large coefficients (numbers in front of formulae). Changing subscripts changes the identity of the substance. For example, changing H 2 O to H 2 O 2 makes hydrogen peroxide, not water.

State two key rules when balancing a chemical equation.

Change only the large coefficients in front of formulae, never the subscripts. Every element must have the same number of atoms on both sides of the equation. Balance elements that appear on their own (such as O 2 ) last .

Balance the equation: __ Al (s) + __ CuO (s) → __ Al 2 O 3 (s) + __ Cu (s)

2 Al (s) + 3 CuO (s) → Al 2 O 3 (s) + 3 Cu (s)

Define relative formula mass ( M r ) .

The relative formula mass ( M r ) of a substance is the total mass of all the atoms in its chemical formula, calculated by adding together the relative atomic masses ( A r ) of every atom present. Each A r must be multiplied by the number of times that atom appears in the formula.

Calculate the M r of Ca(OH) 2 : (1 × __ ) + (2 × __ ) + (2 × __ ) = __

Calculate the M r of Ca(OH) 2 : (1 × 40 ) + (2 × 16 ) + (2 × 1 ) = 74

How many atoms of each element are in Mg(NO 3 ) 2 , and what is its M r ? (Mg = 24, N = 14, O = 16)

Mg(NO 3 ) 2 contains: 1 Mg, 2 N, and 6 O (the subscript 2 outside the bracket multiplies the whole NO 3 group). M r = 24 + (2 × 14) + (6 × 16) = 148 .

True or False? The relative atomic mass ( A r ) is the smaller of the two numbers shown on the Periodic Table.

False. The A r is the larger of the two numbers. The smaller number is the atomic number (the number of protons in the nucleus).

In an open system, how does a gaseous product affect the measured mass of a reaction mixture?

If a gaseous product forms in an open system, it escapes into the atmosphere . The measured mass of the reaction mixture decreases . Mass is still conserved overall: the gas particles have mass and that mass has simply left the measuring system.

Magnesium is heated in air: 2Mg (s) + O 2 (g) → 2MgO (s). Will the mass recorded increase , decrease , or stay the same ? Explain.

The mass will increase . Oxygen gas (a reactant) is absorbed from the air into the system. The only product, magnesium oxide, is a solid that stays in the flask. Because a gaseous reactant has been added from the surroundings, the total recorded mass rises.

If a reaction produces a gaseous product in an open flask, the recorded mass will ________ . If a gaseous reactant is absorbed from the air and all products are solid or liquid, the recorded mass will ________ .

If a reaction produces a gaseous product in an open flask, the recorded mass will decrease . If a gaseous reactant is absorbed from the air and all products are solid or liquid, the recorded mass will increase .

True or False? When a metal carbonate decomposes and the mass decreases, the correct explanation is that the carbonate evaporates.

False. Evaporation is a physical process (liquid to gas). Here, a chemical reaction produces carbon dioxide gas , which then escapes into the atmosphere . The answer must state that a gas is produced and escapes. 'Evaporation' is specifically penalised by mark schemes.

Calcium carbonate decomposes: CaCO 3 (s) → CaO (s) + CO 2 (g). Use the state symbols to predict whether the recorded mass increases or decreases, and explain why.

The mass will decrease . CO 2 is labelled (g) , so it is a gaseous product. In an open system it escapes into the atmosphere. CaO is (s) and stays in the flask. The state symbols show that a gas leaves the system, so the recorded mass falls.

True or False? In a closed reaction vessel, the measured mass always stays constant, even if a gaseous product is formed.

True. In a closed vessel, no gas can escape. All reactants and products remain within the system, so the total measured mass stays constant . This is consistent with the Law of Conservation of Mass.

Distinguish between random errors and systematic errors in an experiment.

Random errors pull a result either too high or too low with no fixed pattern. They can be reduced by repeating measurements and calculating a mean. Systematic errors always pull results in the same direction (always too high or always too low) and are caused by faulty equipment or a flawed experimental procedure.

Give two examples of systematic errors in a chemistry experiment and state the direction of each error.

Not zeroing an electronic balance before use: all mass readings will be too high . Not reading a burette at eye level : all volume readings will be too low . Both errors consistently pull every result in the same direction.

A random error can pull a result in ________ direction. A systematic error always pulls results in the ________ direction. Repeating measurements reduces ________ errors but does not remove ________ errors.

A random error can pull a result in either direction. A systematic error always pulls results in the same direction. Repeating measurements reduces random errors but does not remove systematic errors.

True or False? For an analogue instrument, the uncertainty equals the smallest division on the scale.

False. For an analogue instrument, the uncertainty is half the smallest division (e.g. a thermometer reading to 1°C has uncertainty ±0.5°C). For a digital instrument, the uncertainty equals the smallest division shown on the display.

When suggesting how to improve the reliability of results, why is 'repeat the experiment' an incomplete answer?

Simply repeating the experiment is insufficient because mark schemes require that the mean is also calculated and that anomalous results are excluded . The complete answer is: 'Repeat the experiment and calculate the mean, ignoring any anomalous results.'

For a set of repeated results, the uncertainty = ± ________ of the ________ of the results. This equals the ________ deviation of any single result from the mean.

For a set of repeated results, the uncertainty = ± half of the range of the results. This equals the maximum deviation of any single result from the mean.

A student records three temperatures: 24.5°C, 25.1°C, and 24.8°C. Calculate the mean and the uncertainty .

Mean = (24.5 + 25.1 + 24.8) ÷ 3 = 24.8°C . Range = 25.1 − 24.5 = 0.6°C. Uncertainty = range ÷ 2 = ±0.3°C . Result: 24.8 ± 0.3°C.

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Measurement, Mass & Equations (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards

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FrontConservation of Mass & Balancing Chemical Equations
State the Law of Conservation of Mass.

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Cards in this collection (25)

State the Law of Conservation of Mass.
The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction. The total mass of the reactants always equals the total mass of the products, which is why all chemical equations must be balanced.
A student reacts hydrochloric acid with calcium carbonate in an open flask. The mass of the flask decreases. Explain why.
The reaction produces carbon dioxide gas. In an open flask, this gas escapes into the atmosphere, so its mass is no longer recorded by the balance. Mass is still conserved overall. (Important: do not say evaporation. The gas is produced by a chemical reaction, not a state change.)
If a flask is closed during a reaction, the total mass stays ________.
If a flask is open and a gaseous product forms, the mass will ________ because gas ________ into the atmosphere.
If a flask is closed during a reaction, the total mass stays constant.
If a flask is open and a gaseous product forms, the mass will decrease because gas escapes into the atmosphere.
True or False?
When balancing a chemical equation, you can change the small subscript numbers in a formula to make the equation balance.
False.
You must only change the large coefficients (numbers in front of formulae).
Changing subscripts changes the identity of the substance. For example, changing H₂O to H₂O₂ makes hydrogen peroxide, not water.
State two key rules when balancing a chemical equation.
1. Change only the large coefficients in front of formulae, never the subscripts.
2. Every element must have the same number of atoms on both sides of the equation. Balance elements that appear on their own (such as O₂) last.
Balance the equation:
__Al (s) + __CuO (s) → __Al₂O₃ (s) + __Cu (s)
2Al (s) + 3CuO (s) → Al₂O₃ (s) + 3Cu (s)
Define relative formula mass (M_r).
The relative formula mass (M_r) of a substance is the total mass of all the atoms in its chemical formula, calculated by adding together the relative atomic masses (A_r) of every atom present. Each A_r must be multiplied by the number of times that atom appears in the formula.
Calculate the M_r of Ca(OH)₂: (1 × __) + (2 × __) + (2 × __) = __
Calculate the M_r of Ca(OH)₂: (1 × 40) + (2 × 16) + (2 × 1) = 74
How many atoms of each element are in Mg(NO₃)₂, and what is its M_r? (Mg = 24, N = 14, O = 16)
Mg(NO₃)₂ contains: 1 Mg, 2 N, and 6 O (the subscript 2 outside the bracket multiplies the whole NO₃ group).
M_r = 24 + (2 × 14) + (6 × 16) = 148.
True or False?
The relative atomic mass (A_r) is the smaller of the two numbers shown on the Periodic Table.
False.
The A_r is the larger of the two numbers. The smaller number is the atomic number (the number of protons in the nucleus).
The equation for percentage by mass of an element in a compound is:
$\frac{(__\times number of________of that element)}{__}$ × 100
The equation for percentage by mass of an element in a compound is:
$\frac{(A_{r} \times number of atoms of that element)}{M_{r}}$ × 100
Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃. (N = 14, H = 1, O = 16)
M_r of NH₄NO₃ = (2 × 14) + (4 × 1) + (3 × 16) = 80.
There are 2 nitrogen atoms in the formula.
% N = $\frac{(2 \times 14)}{80}$ × 100 = 35%.
In an open system, how does a gaseous product affect the measured mass of a reaction mixture?
If a gaseous product forms in an open system, it escapes into the atmosphere. The measured mass of the reaction mixture decreases. Mass is still conserved overall: the gas particles have mass and that mass has simply left the measuring system.
Magnesium is heated in air:
2Mg (s) + O₂ (g) → 2MgO (s).
Will the mass recorded increase, decrease, or stay the same? Explain.
The mass will increase. Oxygen gas (a reactant) is absorbed from the air into the system. The only product, magnesium oxide, is a solid that stays in the flask. Because a gaseous reactant has been added from the surroundings, the total recorded mass rises.
If a reaction produces a gaseous product in an open flask, the recorded mass will ________.
If a gaseous reactant is absorbed from the air and all products are solid or liquid, the recorded mass will ________.
If a reaction produces a gaseous product in an open flask, the recorded mass will decrease.
If a gaseous reactant is absorbed from the air and all products are solid or liquid, the recorded mass will increase.
True or False?
When a metal carbonate decomposes and the mass decreases, the correct explanation is that the carbonate evaporates.
False.
Evaporation is a physical process (liquid to gas). Here, a chemical reaction produces carbon dioxide gas, which then escapes into the atmosphere. The answer must state that a gas is produced and escapes. 'Evaporation' is specifically penalised by mark schemes.
Calcium carbonate decomposes:
CaCO₃ (s) → CaO (s) + CO₂ (g).
Use the state symbols to predict whether the recorded mass increases or decreases, and explain why.
The mass will decrease. CO₂ is labelled (g), so it is a gaseous product. In an open system it escapes into the atmosphere. CaO is (s) and stays in the flask. The state symbols show that a gas leaves the system, so the recorded mass falls.
True or False?
In a closed reaction vessel, the measured mass always stays constant, even if a gaseous product is formed.
True.
In a closed vessel, no gas can escape. All reactants and products remain within the system, so the total measured mass stays constant. This is consistent with the Law of Conservation of Mass.
Distinguish between random errors and systematic errors in an experiment.
Random errors pull a result either too high or too low with no fixed pattern. They can be reduced by repeating measurements and calculating a mean.
Systematic errors always pull results in the same direction (always too high or always too low) and are caused by faulty equipment or a flawed experimental procedure.
Give two examples of systematic errors in a chemistry experiment and state the direction of each error.
1. Not zeroing an electronic balance before use: all mass readings will be too high.
2. Not reading a burette at eye level: all volume readings will be too low.
Both errors consistently pull every result in the same direction.
A random error can pull a result in ________ direction.
A systematic error always pulls results in the ________ direction.
Repeating measurements reduces ________ errors but does not remove ________ errors.
A random error can pull a result in either direction.
A systematic error always pulls results in the same direction.
Repeating measurements reduces random errors but does not remove systematic errors.
True or False?
For an analogue instrument, the uncertainty equals the smallest division on the scale.
False.
For an analogue instrument, the uncertainty is half the smallest division (e.g. a thermometer reading to 1°C has uncertainty ±0.5°C). For a digital instrument, the uncertainty equals the smallest division shown on the display.
When suggesting how to improve the reliability of results, why is 'repeat the experiment' an incomplete answer?
Simply repeating the experiment is insufficient because mark schemes require that the mean is also calculated and that anomalous results are excluded. The complete answer is: 'Repeat the experiment and calculate the mean, ignoring any anomalous results.'
For a set of repeated results, the uncertainty = ± ________ of the ________ of the results.
This equals the ________ deviation of any single result from the mean.
For a set of repeated results, the uncertainty = ± half of the range of the results.
This equals the maximum deviation of any single result from the mean.
A student records three temperatures: 24.5°C, 25.1°C, and 24.8°C. Calculate the mean and the uncertainty.
Mean = (24.5 + 25.1 + 24.8) ÷ 3 = 24.8°C.
Range = 25.1 − 24.5 = 0.6°C.
Uncertainty = range ÷ 2 = ±0.3°C.
Result: 24.8 ± 0.3°C.

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