C3 - Quantitive chemistry

Question 1

What is a relative formula mass

Answer 1

• The sum of the relative attic masses of the atoms in the numbers shown in the formula

Question 2

What is the rule for relative formula mass in balanced chemical equations

Answer 2

• The sum of masses of the reactants in the quantities shown equals the sum of the relative formula mass of the products

Question 3

• How can you calculate the % mass of an element in the compound

Answer 3

Percentage mass of an element = Atomic number x number of atoms // relative formula mass

Question 4

• What is the law of mass conservation

Answer 4

• No atoms are lost or made during a chemical reaction so the mass is always equal

Question 5

What does the law of mass conservation mean

Answer 5

• Chemical equations can be represented by a symbol equation which are balanced

Question 6

Why might there seem like there is an increase in mass

Answer 6

• One of reactants is gas that is in the air and products are non gases
• Can’t measure before and its mass isn’t accounted for
• When it reacts it becomes part of it and so the weight increases
• E.g metal and oxygen- mass is metal plus the oxygen

Question 7

Why may the mass may seem to decrease

Answer 7

• One of the products is a gas and products are not
• All are weighed before
• If its not enclosed it can escape and can’t be measured
• e.g Metal carbonate- thermally decomposes to form metal oxide and co2 . Decrease as carbon is not measured

Question 8

• What should any measurement be met with

Answer 8

• Uncertainty

- Can make estimations by plotting

Question 9

What are chemical amounts measured in

Answer 9

• Moles

- Symbol is mol

Question 10

What is the mole equal to

Answer 10

• The mass of one mole of a substance in grams is equal to its relative formula mass

Question 11

How are moles equal

Answer 11

One mole of a substance contains the same number of stated particles, atoms, molecules or ions as any other substance
- All have an equal number

Question 12

The is the avogadro constant

Answer 12

• Number of atoms, molecules or ions in a mole

- Always equals this

Question 13

What are the steps in balancing formula for moles

Answer 13

• Work out relative atomic mass for each of the substances
• Divide actual mass by relative atomic mass- this gives the number of moles
• If not whole numbers-0 divide all by smallest mol value to get whole numbers
• Can write as balanced

Question 14

What are the steps in calculating the mass of reactants and products

Answer 14

• Relative atomic mass times by number of mols gives value in grams

Question 15

What is a limiting reactant

Answer 15

• The reactant that used up before the rest- the rest are in excess
• Directly proportional to amount of product

Question 16

What is done to limit limiting reactions

Answer 16

• Common to use excess of one to ensure all of other reactant is used up

Question 17

How can you calculate the mass of a product by using mass of limiting reactant

Answer 17

• Write out balanced chemical equation
• Work out relative formula mass of reactant and product you want
• Work out how many moles there are in reactant
• Look at ratio between the two and calcite from there

e.g if 5 moles of al react to produce 2.5 mole of aluminium oxide then there will be half the number of moles. So 2.5 x relative formula mass

Question 18

What is concentration

Answer 18

• Measure of how much substance is is a mixed volume

- Measured in grams/dm3

Question 19

How can concentration be measured

Answer 19

• Concentration = mass of solute // volume of solvent