C3 - Quantitative Chemistry

Question 1

Relative Formula Mass -
What is the relative formula mass?

Answer 1

Relative atomic masses of all the atoms in the molecular formula added together.

Question 2

Relative Formula Mass -
What is the symbol for relative formula mass?

Answer 2

Mr

Question 3

Relative Formula Mass -
What is the equation to calculate % by mass of an element in a compound?

Answer 3

Ar x number of atoms of that element
——————————————————- X 100
Mr of the compound

Question 4

The Mole -
What is The Mole?

Answer 4

The name given to an amount of a substance.

Question 5

The Mole -
What is Avagadro’s constant?

Answer 5

6.02 x 10 to the power of 23

Question 6

The Mole -
What is one mole of a substance?

Answer 6

An amount of that substance that contains an Avagadro number of particles - 6.02 x 10 to the power of 23

Question 7

The Mole -
What can the particles in a mole of avagadros constant be?

Answer 7

atoms, molecules, ions or elements

Question 8

The Mole -
Why is the number for Avagadros constant so long?

Answer 8

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass for that substance. eg Carbon has an Ar (relative atomic mass) of 12 and so one mole of carbon weighs exactly 12g

Question 9

The Mole -
What is the equation to calculate the number of moles in a given mass?

Answer 9

      Mr (of element or compound)

Question 10

Conservation of Mass -
What happens to the mass in a chemical reaction?

Answer 10

Mass is always conserved because no atoms are destroyed or created meaning same number + types of atoms on each side of a reaction equation - mass conserved

Question 11

Conservation of Mass -
What is involved if the mass of a chemical reaction appears to have changed?

Answer 11

A gas

Question 12

Conservation of Mass -
Why might the mass of a chemical reaction have appeared to increase?

Answer 12

1. One reactant is a gas (eg oxygen) found in air and all products are solids, liquids or aqueous.
   - Before reaction, gas floating in air not in vessel so can’t account for its mass
   - gas reacts form product, contained in vessel - total mass increase

eg metal + oxygen = metal oxide

Question 13

Conservation of Mass -
Why might the mass of a chemical reaction have appeared to decrease?

Answer 13

1. One of the products is a gas and all reactants are solids, liquids or aqueous.
   - before reaction, all reactant in vessel
   - vessel not enclosed gas escape - can’t account for mass so mass decreases.

eg metal carbonate = metal oxide + carbon dioxide

(gas expands to fill container in, so if not sealed will expand out from vessel and escapes into the air around).

Question 14

The Mole and equations -
What do the large numbers in front of chemical formulas of the reactants and products tell you?

Answer 14

How many moles of each substance takes part/ formed during the reaction

Question 15

The Mole and equations -
What do the little numbers tell you after the chemical formulas of reactant and products tell you?

Answer 15

How many atoms of each element there are in each of the substances

Question 16

The Mole and equations -
How can you balance equations using reacting masses?

Answer 16

1. divide mass each substance by its relative formula mass to find number of moles
2. divide number of moles of each substance by smallest number of moles in each reaction.
3. if any number not integers, multiply all numbers by same amount so become integers.
4. write balanced symbol equation for reaction by putting these numbers in front of the chemical formulae.

Question 17

Limiting reactants -
What is a limiting reactant?

Answer 17

One reactant that is completely used up in a reaction before the rest, the reaction will stop.

Question 18

Limiting reactants -
What are the other reactants that aren’t the limiting reactant in?

Answer 18

excess

Question 19

Limiting reactants -
Why are the other reactants added in excess?

Answer 19

Make sure that the other reactant is used up.

Question 20

Limiting reactants -
Why is it called the limiting reactant?

Answer 20

Limits the amount of product that’s formed.

Question 21

Limiting reactants -
What is the amount of product formed directly proportional to?

Answer 21

The amount of the limiting reactant.

(because if you add more reactant there will be more reactant particles to take part in reaction, means more product particles).

Question 22

Limiting reactants -
What is the mass of the product called in a reaction?

Answer 22

The yield.

Question 23

Limiting reactants -
How can you calculate the mass of a product formed in a reaction by using the mass of limiting reactant and the balanced equation?

Answer 23

1. Write balanced equation
2. Work out Mr of reactant and product wanted
3. find how many moles there are of the substance you know the mass of.
4. use balanced equation work out how many moles of the other substance (in this case how many moles of product made of this many moles of the reactant)
5. use number of moles to calculate mass
   (called theoretical mass)

(also use this method find mass of reactant needed to produce known mass of a product).

Question 24

Gases and solutions -
What does one mole of any gas occupy and at what temperature?

Answer 24

24 dm cubed at 20 degrees celcius

Question 25

Gases and solutions - What will happen to any gas at the same temperature and pressure?

Answer 25

Equal number of moles will occupy the same volume

Question 26

Gases and solutions - What happens to any gas at room temperature and pressure?

Answer 26

One mole will occupy 24 dm cubed.

Question 27

Gases and solutions - What is the equation to calculate the volume of gas (dm cubed) ?

Answer 27

mass of gas (g) ----------------- x 24 Mr of gas

Question 28

Gases and solutions - What is concentration?

Answer 28

The amount of a substance in a certain volume of a solution.

Question 29

Gases and solutions - What happens with the more solute (substance that's dissolved) there is in a given volume to the concentration of a solution?

Answer 29

The concentration of the solution with increase

Question 30

Gases and solutions - What are the way to measure the concentration of a solution?

Answer 30

Calculate mass of substance in given volume of solution.

Question 31

Gases and solutions - What are the two equations to calculate the concentration of a solution?

Answer 31

mass of solute ----------------------- volume of solvent number of moles of solute --------------------------------------- volume of solvent

Question 32

Concentration Calculations - What are titrations?

Answer 32

Experiments let you find volume needed for two solutions to react together completely.

Question 33

Concentration Calculations - How do you find a concentration in g/dm cubed?

Answer 33

By first finding concentration in mol/dm cubed: 1. Calculate Mr of acid 2. convert concentration into concentration in grams by using equation Mass in grams = moles x Mr

Question 34

Atom Economy - What does the atom economy of a reaction tell you?

Answer 34

- How much of mass of reactants is waste when manufacturing a chemical - how much ends up as useful products

Question 35

Atom Economy - What is the equation to calculate atom economy?

Answer 35

Mr of desired products ----------------------------------- x 100 Mr of all reactants

Question 36

Atom Economy - What does 100% atom economy mean?

Answer 36

All atoms in reactants turmed into useful (desired) products.

Question 37

Atom Economy - What does higher atom economy mean?

Answer 37

The greener the process

Question 38

Atom Economy - What are the problems of having a reaction with a low atom economy?

Answer 38

- lots of waste needs to be disposed of - use up resources quickly - unsustainable - not profitable - raw material expensive

Question 39

Atom Economy - What are the reactions with the highest atom economy usually the ones with?

Answer 39

One product (have 100% atom economy)

Question 40

Atom Economy - What does having more products mean?

Answer 40

The more products, the lower the atom economy

Question 41

Atom Economy - What other factors should be considered when choosing which reaction to use to make a certain product?

Answer 41

- yield - rate of reaction - position of equilibrium for reversible reactions (reaction with low atom economy that produces useful by - products also be used)

Question 42

Percentage Yield - What does percentage yield tell you about?

Answer 42

Overall success of an experiment. Compares theoretical and actual yield

Question 43

Percentage Yield - What is the yield?

Answer 43

The amount of product you get.

Question 44

Percentage Yield - How does the amount of reactants you start with affect actual yield?

Answer 44

More reactants, the higher the actual yield

Question 45

Percentage Yield - What does 0% yield mean?

Answer 45

No reactants were converted into products

Question 46

Percentage Yield - What is the equation to calculate percentage yield?

Answer 46

mass of product actually made ---------------------------------------------- x 100 maximum theoretical mass of product

Question 47

Percentage Yield - What are common problems with percentage yield experiments?

Answer 47

1. Not all reactants react to form products - reversible reactions products turn back into reactants so yield never 100% eg Haber process 2. Side effects - reactants sometimes act differently to expected (react with gases in air or impurities in reaction mixture form extra products) 3. Lose product when separate it from reaction mixture. - when filter a liquid to remove solid particles, lose bit of liquid + solid