Amount Of Substance

Question 1

Definition of Avogadro’s constant:

Answer 1

The number of particles (atoms, molecules, ions, electrons) found in one mole of a substance
Value: (6.02 x 10^23)

Question 2

Equation to calculate Number of Particles:

Answer 2

Number of particles = (6.02 × 10^23) × Moles

Question 3

Definition of Relative Atomic Mass (Ar):

Answer 3

Ar = Average mass of 1 atom of an element ÷ (1/12th mass of a Carbon-12 atom)

Question 4

Definition of Relative Molecular Mass (Mr):

Answer 4

Mr = Average mass of one molecule ÷ 1/12th mass of a Carbon-12 atom

Question 5

Equation to calculate Moles:

Answer 5

Moles = Mass ÷ Mr

Question 6

How to calculate moles in chemical equations:

Answer 6

1) Ensure reaction equation is balanced
2) Calculate moles of one substance using information given
3) Use ratios in balanced equation to calculate moles of other substances

Question 7

Equation for Percentage Yield:

Answer 7

% Yield = 100 × (Actual Yield) ÷ Theoretical Yield)

Question 8

Why is it desirable to have a high percentage yield?

Answer 8

-Increases amount of products made from reactants
-% Yield can be increased by making improvements in experimental techniques

Question 9

Definition of Actual Yield:

Answer 9

The actual yield is the amount of product obtained in reality in the reaction. The actual yield is always smaller than the theoretical yield

Question 10

Definition of Theoretical Yield:

Answer 10

The theoretical yield is the quantity of the product that should be obtained in the reaction if it goes to completion. It is the maximum product you could get.

Question 11

Why is the actual yield always smaller than the theoretical yield?

Answer 11

1) Some of the product could be lost
2) Some reactants don’t react
3) Waste products could be made

Question 12

Definition for Atom Economy:

Answer 12

Measure of how much of the reactants is being converted into the desired products

Question 13

Equation for Atom Economy:

Answer 13

% = 100 × (Mass of desired products) ÷ (Total mass of reactants)

Question 14

Why is it desirable to have a high atom economy?

Answer 14

1) Reduces amount of waste-by products
2) Increases mass of atoms in reactants that are turned into products

Question 15

Equation for Concentration:

Answer 15

Concentration (mol dm-3) = Moles ÷ Volume

Question 16

What is a titration?

Answer 16

To use one chemical solution of a known concentration (standard solution) to determine concentration of another solution by reacting two solutions together

Question 17

What is the titre in a titration?

Answer 17

The volume of solution added from the burette needed for complete reaction with the solution in the conical flask

Question 18

Definition of concordant titres:

Answer 18

Titres that are within 0.1cm3 of each other
Concordant titres are used to calculate concentration of other solution

Question 19

Rules for Titration results:

Answer 19

-All values must be recorded to 2dp
-The 2nd decimal place can only be 0 or 5 because the burette can not read more accurately than this

Question 20

Steps to make up a Standard Solution: Stage 1

Answer 20

Stage 1: Transfer known mass of solid
a) weigh sample bottle containing solid on a 2dp balance
b) transfer to beaker and reweigh sample bottle
c) record difference in mass

Question 21

Steps to make up a Standard Solution: Stage 2

Answer 21

Stage 2: Dissolve in water
a) add distilled water
b) stir with a glass rod
c) until all solid has dissolved

Question 22

Steps to make up a Standard Solution: Stage 3

Answer 22

Stage 3: Transfer, washing and agitation
a) transfer to volumetric flask using a funnel
b) with washings
c) make up to 250cm3 mark with water
d) shake/invert/mix

Question 23

How to improve titration accuracy:

Answer 23

1) Remove air bubbles from burette tap - volume of air bubble will be counted in burette reading
2) Remove funnel used to fill burette before starting titration - solution drops could fall from funnel into burette and affect burette reading
3) During titration, swirl conical flask and rinse walls of conical flask with distilled water - ensures all reactants mix and no reagent is left unreacted on sides of flask

Question 24

Equation for Percentage Error:

Answer 24

% error = 100 × (Error of equipment) ÷ (Amount measured)

Question 25

How to reduce percentage error:

Answer 25

1) Increase concentration of solution in conical flask
2) Decrease concentration of solution in burette

Question 26

Equation for ideal gas:

Answer 26

PV = nRT

pressure (Pa) × volume (m3) = moles of gas (n) × molar gas constant (8.31Jk-1mol-1) × temperature (K)

Question 27

Conversions for Volume in PV=nRT

Answer 27

1m3 = 1,000,000 cm3
1m3 = 1000 dm3

Question 28

Definition of Empirical formula:

Answer 28

Simplest whole number ratio of atoms of each element

Question 29

Definition of Molecular formula:

Answer 29

Actual number of atoms of each element