Chemical formulae, equations, calculations (Edexcel IGCSE Chemistry (Modular): Unit 1): Flashcards

The M_r of a substance is calculated by adding the relative atomic masses (A_r) of all the atoms in the formula. You must account for how many times each atom appears in the formula.

M_r is the symbol for relative formula mass. It equals the total mass of all the atoms in a chemical formula, found by adding up the A_r values of every atom present.

To calculate the M_r of H₂O, you add 2 × A_r(H) to 1 × A_r(O), giving an M_r of 18.

True.

The M_r of NaCl is 58.5. This is calculated as A_r(Na) + A_r(Cl) = 23 + 35.5 = 58.5.

The relative atomic mass is the larger of the two numbers shown for each element on the Periodic Table. It appears above or below the element symbol, depending on the table layout.

Complete the M_r values for these substances.

True.

This follows from the law of conservation of mass. Atoms are not created or destroyed, so the total relative formula mass is conserved across the reaction.

A mole is the unit used to measure chemical amounts. One mole of any substance contains the same number of particles — 6.02 × 10²³ — as one mole of any other substance.

The Avogadro constant is the number of particles in one mole of any substance. Its value is 6.02 × 10²³ per mole.

True.

One mole of any element contains exactly 6.02 × 10²³ atoms. This number is the Avogadro constant.

Molar mass is the mass of one mole of a substance, measured in g mol^-1. For an element it equals its A_r in grams; for a compound it equals its M_r in grams.

To find the number of moles, divide the mass by the molar mass. To find the mass, multiply the moles by the molar mass.

The mass of 0.250 moles of zinc is found by moles × molar mass = 0.250 × 65.38 = 16.3 g.

False.

The M_r of CO₂ is 44, not 48. It is calculated as A_r(C) + 2 × A_r(O) = 12 + (2 × 16) = 44.

The number of moles is calculated by dividing the mass by the molar mass. The formula is: moles = mass ÷ molar mass.

Complete the particle type for each substance.

The three steps for calculating the mass of a product are:

1. Calculate the moles of the given substance (moles = mass ÷ M_r)

2. Use the molar ratio from the balanced equation to find moles of the required substance

3. Calculate the mass of the required substance (mass = moles × M_r)

In a reacting mass calculation, divide mass by the molar mass (or M_r) to find moles, and multiply moles by the molar mass (or M_r) to find mass.

True.

The molar ratio is read directly from the coefficients in the balanced equation. It tells you how many moles of each substance react or are produced relative to the others.

To write a balanced equation from experimental masses, convert each mass to moles by dividing by the M_r. Then simplify the mole values to find the smallest whole number ratio, and use these as the coefficients in the equation.

A molar ratio shows the relative number of moles of each substance that reacts or is produced. It is given by the coefficients (large numbers) in the balanced chemical equation.

True.

Moles of Mg = 6.0 ÷ 24 = 0.25 mol. The molar ratio is 1:1, so 0.25 mol of magnesium oxide is produced. Mass = 0.25 × 40 = 10 g.

Complete the table by calculating the number of moles.

You can work in any mass unit (grams, tonnes, kilograms) because the reacting masses are always in proportion to the balanced equation. As long as you are consistent throughout the calculation, the ratios remain correct.

The actual yield is the measured amount of product obtained from a chemical reaction in practice. It is always less than or equal to the theoretical yield.

The percentage yield is rarely 100% because:

1. Some product may be lost in equipment or during transfer

2. The reaction may be reversible, so products convert back to reactants

3. Products can be lost during separation stages such as filtration or distillation

The theoretical yield is the maximum amount of product that could be obtained if the reaction went to completion with no losses. It is calculated from the balanced equation and the reacting masses.

Percentage yield = (actual yield ÷ theoretical yield) × 100.

False.

A percentage yield greater than 100% is impossible. If your calculation gives a value above 100%, you have likely divided the theoretical yield by the actual yield instead of the other way around.

The percentage yield = (actual yield ÷ theoretical yield) × 100 = (1.6 ÷ 2.0) × 100 = 80%.

A reaction pathway is a sequence of reactions used to produce a required product. Different pathways can be compared and evaluated, with percentage yield being a key factor in choosing the best method.

False.

A reversible reaction can never achieve 100% yield, because the products continuously turn back into reactants. The forward and reverse reactions occur simultaneously.

Hydrated salts are crystallised salts that contain water of crystallisation as part of their structure. Their formula includes a • symbol to show the water, e.g. CuSO₄•5H₂O.

Finding the formula of a simple compound involves measuring masses before and after a reaction, then converting masses into moles. The molar ratio of reactants and products gives the empirical formula.

True.

Hydrated copper sulfate is blue. Heating drives off the water of crystallisation, turning the salt white (anhydrous). Heating stops when the colour change is complete.

In the experiment to find the formula of a hydrated salt, the mass of water is calculated by subtracting the mass of the anhydrous salt from the mass of the original hydrated sample.

Heating must be gentle because overheating can decompose the salt itself, causing a larger mass change than expected. This would give an inaccurate result for the amount of water lost.

Water of crystallisation is water that is chemically bonded within the crystal structure of a hydrated salt. It is shown after the • symbol in the formula, e.g. CuSO₄•5H₂O.

The aim is to determine the empirical formula of magnesium oxide by burning magnesium in air inside a crucible.

True.

Heating continues until a constant mass is reached. This confirms that the reaction is complete and all the magnesium has been converted to magnesium oxide.

The lid is lifted frequently to allow enough air (oxygen) into the crucible for the magnesium to fully oxidise. The lid is replaced quickly to prevent magnesium oxide smoke from escaping, which would cause a loss of mass.

To find the empirical formula of magnesium oxide, the mass of each element is divided by its relative atomic mass to find the number of moles, and the moles are then simplified to give the simplest whole number ratio.

The mass of oxygen is calculated by subtracting the mass of magnesium used from the mass of magnesium oxide formed. The magnesium oxide mass is found by subtracting the empty crucible mass from the final total mass.

True.

Methane acts as the reducing agent, removing oxygen from copper(II) oxide to leave copper metal. Excess methane is burned off to prevent a build-up of flammable gas.

The colour change from black (copper(II) oxide) to pink/orange (copper metal) shows that all the oxygen has been removed. This means the reduction is complete.

An empirical formula is the simplest whole number ratio of the atoms of each element present in a compound. For example, the empirical formula of ethanoic acid (C₂H₄O₂) is CH₂O.

A molecular formula shows the actual number and type of each atom in a molecule. An empirical formula shows only the simplest whole number ratio of atoms. These are often different for organic molecules.

True.

Ionic compounds do not exist as discrete molecules, so their formula always represents the simplest ratio of ions. For example, NaCl is the empirical formula of sodium chloride.

To calculate an empirical formula, divide the mass of each element by its relative atomic mass to get moles, then divide all moles by the smallest number of moles.

Divide the relative formula mass (M_r) of the molecular formula by the M_r of the empirical formula. Multiply the number of each atom in the empirical formula by this value to get the molecular formula.

A molecular formula shows the actual number and type of each atom present in one molecule of a compound. For example, the molecular formula of ethane is C₂H₆, while its empirical formula is CH₃.

False.

You must multiply all ratios by a suitable number (in this case 2) to make them whole numbers. Rounding just one ratio without adjusting the others gives the wrong formula.

If the ratio is not a whole number (e.g. 1.5), multiply all the ratios by a suitable whole number to convert them. For example, multiply by 2 if the ratio contains 1.5, or by 3 if it contains 1.33 or 1.67.

Find the mass of the anhydrous salt and the mass of water lost on heating. Divide each by its M_r to get moles, then find the simplest ratio of salt to water. Write the formula as salt•xH₂O.

A solute is a solid substance that dissolves in a liquid (solvent) to form a solution. The amount of solute can be expressed in grams (g) or moles (mol).

Two units for concentration are g / dm³ and mol / dm³. The mol / dm³ unit is more useful to a chemist because it relates to the number of moles of solute dissolved.

True.

There are 1000 cm³ in 1 dm³, so dividing by 1000 converts between the two units. You must always use dm³ in the concentration formula.

Concentration (in mol / dm³) is calculated by dividing the number of moles of solute by the volume of solution in dm³.

A solution is a mixture formed when a solute dissolves in a solvent. The volume of a solution is measured in cm³ or dm³.

Multiply the concentration (in mol / dm³) by the volume (in dm³). Make sure the volume is converted from cm³ to dm³ by dividing by 1000 before substituting.

True.

A higher concentration means more solute is dissolved in the same volume of solution. A solution of 4 mol / dm³ contains twice as many moles of solute per dm³ as one of 2 mol / dm³.

First convert the mass to moles by dividing by the M_r of the solute. Then divide the moles by the volume of solution in dm³ to get the concentration in mol / dm³.

Avogadro's Law states that at the same temperature and pressure, equal amounts (in moles) of any gas occupy the same volume. This means gas volumes are directly proportional to the number of moles.

The molar gas volume at RTP is 24 dm³ per mole (or 24,000 cm³ per mole). RTP stands for room temperature and pressure: 20 °C and 1 atmosphere (atm).

False.

By Avogadro's Law, equal numbers of moles of any gas occupy the same volume at the same temperature and pressure. Both would occupy 2 × 24 = 48 dm³ at RTP.

The volume of a gas at RTP is calculated by multiplying the number of moles by the molar gas volume of 24 dm³ per mole.

First convert the mass to moles by dividing by the M_r of the gas. Then multiply moles by the molar gas volume (24 dm³ or 24,000 cm³) to find the volume at RTP.

The molar gas volume is the volume occupied by one mole of any gas at a specified temperature and pressure. At RTP (20 °C, 1 atm), the molar gas volume is 24 dm³.

False.

A 1:5 molar ratio means 5 volumes of oxygen are needed for every 1 volume of propane. So 150 cm³ of propane requires 5 × 150 = 750 cm³ of oxygen.

To find gas volumes from a balanced equation, use the molar ratios as volume ratios. Multiply the volume of the known gas by the ratio from the equation to find the unknown volume.

You must check units because the molar gas volume is 24 dm³ or 24,000 cm³. Using the wrong value gives an answer in the wrong units. Always match the molar volume to the units asked for in the question.