Reaction Calculations (Edexcel A Level Chemistry): Flashcards
Exam code: 9CHO
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What is molar gas volume?
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What is molar gas volume?
The molar gas volume is the volume occupied by one mole of any gas under a specified set of conditions.
At r.t.p. (293 K / 20 °C, 100 kPa), the molar gas volume is 24.0 dm3 mol-1.
At s.t.p. (273 K, 101.3 kPa), the molar gas volume is 22.4 dm3 mol-1.
What is the equation used to calculate the number of moles of a gas from its volume?
The number of moles of a gas is calculated using:
number of moles = volume of gas (dm3) / molar gas volume (dm3 mol-1)
At r.t.p., the molar gas volume is .......... dm3 mol-1. At s.t.p., the molar gas volume is .......... dm3 mol-1.
At r.t.p., the molar gas volume is 24.0 dm3 mol-1. At s.t.p., the molar gas volume is 22.4 dm3 mol-1.
True or False?
Standard temperature and pressure (s.t.p.) is defined as 20 °C and 101.3 kPa.
False.
S.t.p. is defined as 0 °C (273 K) and 101.3 kPa. It is r.t.p. that uses 20 °C (293 K).
What is an ideal gas?
An ideal gas is a gas that perfectly obeys the kinetic theory of gases, with molecules that have negligible volume, no intermolecular forces, undergo elastic collisions and whose temperature is proportional to their average kinetic energy.
What does each symbol represent in the ideal gas equation PV = nRT?
In the ideal gas equation PV = nRT:
P = pressure (Pa), V = volume (m3), n = moles of gas (mol), R = gas constant (8.31 J K-1 mol-1), T = temperature (K).
The ideal gas equation is P.......... = nRT, where pressure is measured in .......... and volume is measured in .......... .
The ideal gas equation is PV = nRT, where pressure is measured in pascals (Pa) and volume is measured in m3.
True or False?
According to kinetic theory, ideal gas molecules have negligible volume and no intermolecular forces.
True.
The kinetic theory of gases assumes that ideal gas molecules have negligible volume and experience no intermolecular forces, unlike real gases which deviate from this behaviour.
How do you convert a temperature from °C to kelvin?
To convert from °C to kelvin, add 273 to the Celsius value.
For example, 25 °C = 298 K.
What is a standard solution?
A standard solution is a solution of precisely known concentration, prepared using an accurately weighed mass of solute and a volumetric flask to ensure the volume is exact.
What is the end point of a titration?
The end point (or equivalence point) is the point at which the two solutions have completely reacted.
It is detected by a permanent colour change of the indicator added to the solution in the conical flask.
The formula for concentration is:
concentration (mol dm-3) = number of moles (mol) / .......... (dm3)
The formula for concentration is:
concentration (mol dm-3) = number of moles (mol) / volume (dm3)
True or False?
Concordant titration results are within 0.10 cm3 of each other.
True.
Concordant results are defined as titre values that are within 0.10 cm3 of each other. Only concordant results are averaged; non-concordant results are discarded.
What is volumetric analysis?
Volumetric analysis is a technique that uses the volume and concentration of one reactant (a standard solution) to determine the concentration of another unknown solution, most commonly by performing a titration.
A burette is marked to a precision of 0.10 cm3, so its uncertainty is recorded as .......... cm3. The uncertainty on a titre (calculated from two readings) is therefore .......... cm3.
A burette is marked to a precision of 0.10 cm3, so its uncertainty is recorded as ±0.05 cm3. The uncertainty on a titre (calculated from two readings) is therefore ±0.10 cm3.
True or False?
A white tile is placed under the burette during a titration to make the colour change easier to see.
False.
The white tile is placed under the conical flask, not the burette. This makes the colour change of the indicator easier to detect against the white background.
What are the steps for calculating the concentration of an unknown solution from titration data?
To calculate the concentration of an unknown solution:
Write the balanced equation and identify the molar ratio.
Calculate the moles of the known solution: moles = concentration x volume (in dm3).
Use the molar ratio to find moles of the unknown.
Calculate concentration = moles / volume (in dm3).
Why must cm³ be converted to dm³ when calculating moles from concentration?
The concentration equation uses volume in dm3 (mol dm-3 x dm3 = mol).
To convert cm3 to dm3, divide by 1000. For example, 25.0 cm3 = 0.0250 dm3.
What is a random error?
A random error is an unpredictable variation in a measurement that causes readings to be scattered above and below the true value.
Random errors can be reduced by repeating measurements and calculating the mean.
What is a systematic error?
A systematic error is an error that consistently shifts all measurements in the same direction, always producing values that are too high or too low.
Systematic errors cannot be reduced by repeating the experiment.
Percentage uncertainty = (absolute uncertainty / .......... ) x 100
Percentage uncertainty = (absolute uncertainty / measured value ) x 100
True or False?
Repeating an experiment and averaging the results will reduce a systematic error.
False.
Systematic errors always pull results in the same direction, so averaging does not remove them. Repeating and averaging reduces the effect of random errors instead.
What is the difference between percentage uncertainty and percentage error?
Percentage uncertainty compares the absolute uncertainty to the measured value.
Percentage error compares an experimental result to a literature (accepted) value. They measure different things and should not be confused.
When multiplying or dividing measurements, you add together the .......... uncertainties. When adding or subtracting measurements, you add together the .......... uncertainties.
When multiplying or dividing measurements, you add together the percentage uncertainties. When adding or subtracting measurements, you add together the absolute uncertainties.
True or False?
Taking repeated measurements and calculating the mean reduces the effect of random errors.
True.
Random errors cause readings to scatter above and below the true value, so taking the mean of repeated measurements helps cancel out these fluctuations.
When a measurement requires two readings from the same instrument, how is the uncertainty calculated?
The total absolute uncertainty is found by adding the individual uncertainties from each reading.
For example, if a burette has an uncertainty of ±0.05 cm3 per reading, two readings give a total uncertainty of ±0.10 cm3.
Give two examples of a systematic error that could occur in a school chemistry experiment.
Examples of systematic errors include:
Failing to zero an electronic balance before use, causing all mass readings to be too high.
Not reading a burette at eye level, leading to a consistent parallax error where volumes appear smaller than they are.
What is percentage yield?
Percentage yield is a measure of how much of a desired product is obtained experimentally compared to the maximum theoretical yield, expressed as a percentage.
percentage yield = (actual yield / theoretical yield) x 100
What is atom economy?
Atom economy is a measure of how efficiently atoms from the reactants are incorporated into the desired product, calculated from the balanced equation using molar masses.
atom economy = (molecular mass of desired product / sum of molecular masses of all products) x 100
percentage yield = (.......... yield / .......... yield) x 100
percentage yield = (actual yield / theoretical yield) x 100
Give three reasons why the percentage yield of a reaction is rarely 100%.
Percentage yield is rarely 100% because:
Other reactions occur simultaneously, consuming reactants.
The reaction does not go to completion.
Products are lost during separation and purification steps.
True or False?
Addition reactions always have an atom economy of 100%.
True.
In addition reactions, all atoms from the reactants are incorporated into a single product, so no atoms are wasted and atom economy = 100%.
atom economy = (molecular mass of .......... product / sum of molecular masses of .......... products) x 100
atom economy = (molecular mass of desired product / sum of molecular masses of all products) x 100
What is the difference between actual yield and theoretical yield?
The actual yield is the mass (or moles) of product obtained from the experiment.
The theoretical yield is the maximum mass (or moles) of product calculated from a reacting mass calculation, assuming complete reaction with no losses.
True or False?
A calculated percentage yield greater than 100% always indicates an accurate experimental result.
False.
A percentage yield above 100% indicates an error in the experiment. Possible causes include crystals that are still wet when weighed, impurities in the product or the mass of a container being included in the product mass.
How does atom economy differ from percentage yield?
Atom economy is calculated from the balanced equation alone and measures how much of the reactant mass becomes the desired product regardless of how much is actually made.
Percentage yield measures how much of the theoretical maximum was obtained experimentally, so it depends on reaction conditions and losses.
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