Moles

Question 1

What is the relative atomic mass (Ar)

Answer 1

The average mass of an element relative to the mass of 1/12 of a carbon-12 atom

Question 2

What is the relative molecular mass (Mr)

Answer 2

The sum of the RAMs of the atoms making up a molecule

Question 3

What is the relative formula mass (Mr)

Answer 3

If the compound contains ions the relative formula mass is the mass of the formula unit relative to the mass of 1/12 of an atom of carbon-12

Question 4

Which number must you always use when calculating moles - the big or small on periodic table

Answer 4

Big - atomic mass

Question 5

What is the mass of 1 mol of a substance in grams

Answer 5

The relative molecular mass

Question 6

What is the Avogadro Constant and what is it for

Answer 6

6.02x10^23 - the number of atoms present in 1 mol of any substance

Question 7

What equation links number of moles, mass and mass of one mole

Answer 7

Mass = mass of one mole x number of moles

Question 8

How do you work out the % by mass of an element present in a compound

Answer 8

(RAM x number of atoms of that element)/RAM of the whole compound

Question 9

What is the empirical formula

Answer 9

The simplest whole number ratio of the elements present

Question 10

What is the molecular formula

Answer 10

The total number of atoms of each element present in a molecule of the compound. The normal one

Question 11

How to find the empirical formula

Answer 11

Divide each mass by its RAM
2. Divide all of those values by the smallest of them to get the smallest ratio

Question 12

How are moles mostly used - use equation 2Mg + O2 -> 2MgO

Answer 12

To calculate the amounts of reactants and products in chemical reactions.

The ratio of each part

2 parts Mg + one part O2 -> 2 parts MgO
2:1:2
Could be multiplied though e.g. 4:2:4

Question 13

How to answer most moles question (3)

Answer 13

1. Work out the number of moles of anything you can
2. Use the chemical equation to work out the number of moles of the substance you require
3. Convert the number of moles into the required quantity e.g. mass

Question 14

Describe an experiment to determine the volume of gas produced when 1g of various carbonates are reacted with excess acid

Answer 14

1. Set up apparatus to collect the gas over water using a 250ml measuring cylinder and delivery tube
2. Weigh out 1g of one of the metal carbonates
3. Put 25cm^3 of 1mol/dm^3 hydrochloric acid (excess) into a conical flask and weigh the conical flask
4. Add the metal carbonate to the conical flask and quickly put the bung over the tube
5. Record the volume of gas collected and the final mass of the conical flask + contents
6. Repeat the process with a different carbonate

Question 15

What is 1dm^3

Answer 15

1,000cm^3

Question 16

What is the volume of 1 mol of gas at room temperature and pressure

Answer 16

24dm^3

Question 17

What is the equation linking gas volume, number of moles and molar volume

Answer 17

Gas volume = number of moles x molar volume (24)

Question 18

How to work out percentage yield

Answer 18

Actual yield/theoretical yield x 100

Question 19

What is water of crystallisation

Answer 19

When some substances crystallise, water molecules are incorporated into the crystal lattice.

Question 20

What are hydrated substances and what are anhydrous

Answer 20

Hydrated - crystallised structures that contain water of crystallisation

Anhydrous - do not contain them

Question 21

How do you determine the number of moles of water of crystallisation

Answer 21

1. Work out how many moles of water are present
2. Work out how many moles of the other substance e.g. copper sulfate are present
3. Work out the ratio

The formula is in the form: CuSO4.5H2O - take note of the dot before the water