Topic C3 Revision

Question 1

What is the abbreviation of relative formula mass?

Answer 1

Mr

Question 2

How do you calculate Mr (relative formula mass)

Answer 2

You add all the relative atomic masses (Ar) in the molecular formula.

Question 3

Where do you find the Ar (atomic mass) of an element?

Answer 3

It is the bigger number on the periodic table, but on some exam questions they give it to you.

Question 4

What is the formula for calculating the percentage mass of an element in a compound?

Answer 4

Percentage mass of an element in a compound
.Ar× num of atoms of that element
_________________________ ×100
Mr of the compound

Question 5

What is a mole?

Answer 5

One mole of any substance is an amount of that substance that contains an Avogadro number of particles (6.02 × 10^23).

One mole of any substance will have a mass in grams equal to the relative Ar or Mr of the element or compound.

Question 6

Oxygen has an atomic mass of 16. How much would one mole of oxygen weigh?

Answer 6

16g

Question 7

Carbon dioxide has a relative formula mass of 44. How much would one mole of carbon dioxide weigh?

Answer 7

44g

Question 8

What is the formula to find the number of moles in a given mass?

Answer 8

Number of moles (n) = mass (m) in g / Mr

Question 9

What mass of carbon is there in 4 moles of carbon dioxide?

Mr of carbon = 12

Answer 9

M = n × Mr
= 4 × 12
=48

There are 48g of carbon in 4 moles of carbon dioxide

Question 10

What is the conservation of mass?

Answer 10

In a chemical reaction, mass is always conserved. That means that the mass of the reactants is always equal to the mass of the products.

Question 11

Why might the mass appear to change in a chemical reaction?

Answer 11

If the mass increases, it’s probably because it has reacted with a gas found in the air, and all of its products are solid, liquid or aqueous, and can therefore be measured.

If the mass decreases, it’s probably because one of the products is a gas, and therefore couldn’t be weighed.

Question 12

What do the big numbers in front of the chemical formulas tell you? E.g. 2HCL

Answer 12

They tell you how many moles of each substance takes part or is formed by the reaction.
E.g. If it says 2HCl there is 2 moles of HCl in the equation.

Question 13

What do the small numbers after the symbol mean?

Answer 13

How many atoms of each element there are in a substance.

Question 14

what is the avogadro constant?

Answer 14

6.02 x 10^23 (the number of particles of a substance it takes for the substance to have exactly the same weight in grams as its relative atomic or formula mass)

Question 15

if you know the masses of the reactants and products in a reaction, what are the 4 steps to working out the balanced symbol equation for the reaction using moles?

Answer 15

1. divide the mass of each substance by its relative formula mass to find the number of moles
2. divide the number of moles of each substance by the smallest number of moles in the reaction (i.e. the number of moles of the substance with the lowest number of moles)
3. if any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers
4. write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas

Question 16

when do reactions stop?

Answer 16

when all of one of the reactants is used up - this reactant is called the limiting reactant

Question 17

why would you add one reactant in excess?

Answer 17

to make sure that all of the other reactants are used up

Question 18

what is the relationship between the amount of product formed in a reaction and the amount of limiting reactant? Why is this?

Answer 18

they are directly proportional, because if you add more reactant there will be more reactant particles to take part in the reaction, which means more product particles

Question 19

what are the steps to calculating the mass of a product formed in a reaction by using the mass of the limiting reactant and the balanced symbol equation? What is another use of this method?

Answer 19

1. work out the balanced equation
2. work out the relative formula masses (Mr) of the reactant and product you want
3. find out how many moles there are of the substance you know the mass of
4. use the balanced equation to work out how many moles there’ll be of the other substance. In this case, that’s how many moles of product will be made of this many moles of reactant.
5. Use the number of moles to calculate the mass
6. this method could also be used to find the mass of a reactant needed to produce a known mass of product

Question 20

what is concentration a measure of?

Answer 20

how much of a substance (e.g. the mass or the number of moles) can be found in a certain volume of a solution

Question 21

what is the solute?

Answer 21

the substance that’s dissolved

Question 22

give a unit of concentration

Answer 22

g/dm^3

Question 23

give one way to measure the concentration of a solution

Answer 23

by calculating the mass of a substance in a given volume of solution. The units would be units of mass/units of volum

Question 24

give the formula to find the concentration of a substance (include example units)

Answer 24

concentration (g/dm^3) = mass of solute (g) / volume of solvent (dm^3)