1.2 - Amounts of Substance

Question 1

What is Avogadro’s Constant?

Answer 1

6.02 x 10^23

Question 2

What is the formula that links together the number of particles, Avogadro’s Constant and the number of moles?

Answer 2

Number of Particles = Number of Moles x Avogadro’s Constant

Question 3

How heavy is one mole of a substance?

Answer 3

One mole of a substance is equal to its relative molecular mass

Question 4

What is the formula that links together the number of moles, relative molecular mass and the mass of a substance?

Answer 4

Number of Moles = Mass of a Substance / Relative Molecular Mass

Question 5

How many moles of Al2O3 are present in 5.10 grams?

Answer 5

Relative Molecular Mass = (2x27) + (3 x 16) = 102

5.10 / 102 = 0.05 moles

Question 6

What is the formula that links together the number of moles, volume and concentration?

Answer 6

Number of Moles = Concentration x Volume (dm3)

Question 7

How many moles of lithium chloride is present in 25cm3 of a 1.2 mol / dm3 solution of LiCl?

Answer 7

Moles = 1.2 x (25 / 1000) = 0.03

Question 8

What is the formula for the Ideal Gas Equation?

Answer 8

pV = nRT

p = pressure (Pa)
V = volume (m3)
n = number of moles
R = gas constant (8.31)
T = temperature (K)

Question 9

How many moles are there in 0.06 m3 of hydrogen gas, at 283K and 50,000 Pa?

Answer 9

n = (pV) / (RT)
n = (50,000 x 0.06) / (8.31 x 283) 
n = 1.28 moles

Question 10

What is 1 kPa in Pa?

Answer 10

1,000 Pascals

Question 11

How do you convert between degrees and K?

Answer 11

Add 273

Question 12

How many cm3 is in an m3?

Answer 12

1,000,000 cm3

Question 13

How many dm3 is in an m3?

Answer 13

1,000 dm3

Question 14

What are the three steps in writing an ionic equation?

Answer 14

1) Write a full balanced equation for the reaction
2) Split any dissolved ionic species into ions
3) Take out any ions that appear on both sides of the equation

Question 15

What are the four steps to calculate the reaction to work out how much product you will get from a certain mass of the reactant?

Answer 15

1) Write out the balanced equation for the reaction
2) Work out how many moles of the reactant you have
3) Use the molar ratio from the balanced equation to work out the number of moles of product that will be formed from this much reactant
4) Calculate the mass of that many moles of product

Question 16

What reaction happens when an acid reacts with an alkali to produce salt and water?

Answer 16

A neutralisation reaction

Question 17

What is a standard solution?

Answer 17

A standard solution is any solution that you know the exact concentration of. Making a standard solution involves dissolving a known amount of solid in a known amount of water to create a known concentration

Question 18

How do you work out how much solid needs to be used to make a standard solution?

Answer 18

1) Work out the number of moles needed using the formula moles = concentration x volume
2) Work out how many grams are needed using the formula mass = moles x relative molecular mass

Question 19

How is the exact mass of solid measured?

Answer 19

1) Place a weighing bottle on a digital balance. Weight out the required mass of solid approximately and tip it into a beaker
2) Weigh the weighting bottle (which may contain traces of the solid). Subtract the mass of the bottle from the mass and the bottle together to find the precise mass of solid you have weighed out

Question 20

How do you make sure no solute is on the rod, beaker and funnel?

Answer 20

They are all rinsed with distilled water and added to the standard solution

Question 21

How is the correct amount of water measured?

Answer 21

The bottom of the meniscus has to reach the line. When you get close to the line, add the water dropwise

Question 22

How do you make the standard solution well mixed?

Answer 22

Stopper the flask and turn it upside down a few times

Question 23

What is the purpose of titration?

Answer 23

Titrations allow you to work out exactly how much acid is needed to neutralise a measured quantity of alkali (or the other way around)

Question 24

What are concordant results?

Answer 24

For titrations, concordant results are results within 0.1 cm3 of each other

Question 25

What is the empirical formula?

Answer 25

The empirical formula gives the smallest whole-number ratio of atoms of each element present in a compound

Question 26

What is the molecular formula?

Answer 26

The molecular formula gives the actual numbers of atoms in a molecule

Question 27

How are empirical formulas calculated?

Answer 27

1) Assume you have 100g of the compound - you can turn the percentages straight into masses.
2) Work out how many moles of each element are in 100g of the compound
3) Divide each number of moles by the smallest number of moles. This gives you the ratio of the elements in the compound
4) Apply these numbers from the ratio in the formula

Question 28

What is ‘theoretical yield’?

Answer 28

Theoretical yield is the mass of product that should be formed in a chemical reaction - it assumes no chemicals are ‘lost’ in the process

Question 29

How is the theoretical yield calculated?

Answer 29

1) Work out how many moles of the reactant you have
2) Use the equation to work out how many moles of product you would expect that that much reactant to make
3) Calculate the mass of that many moles of product

Question 30

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

Answer 30

Sometimes all of the starting chemicals don’t react fully
Some chemicals are ‘lost’ e.g. some solution gets left on the filter paper
Some are lost during transfers between containers
Other products are formed in side reactions

Question 31

What is the equation that links together percentage yield, actual yield and theoretical yield?

Answer 31

Percentage Yield = (Actual Yield / Theoretical Yield) x 100

Question 32

What is atom economy?

Answer 32

Atom economy is the measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation

Question 33

What are the advantages of a high atom economy?

Answer 33

More efficient use of raw material
Less waste to deal with (especially if they are toxic)
Less money spent on separation
Better for the environment
Sustainable
Companies can sell the product for cheaper and make it more widespread

Question 34

What is the equation that links together atom economy, the molecular mass of desired products and the sum of the molecular masses of all the reactants?

Answer 34

Atom Economy = (Molecular Mass of Desired Products / Sum of the Molecular Masses of all the Reactants) x 100