Quantitative Chemistry

Question 1

What does the formula H2SO tell you?

Answer 1

There are 3 elements: hydrogen, sulfur and oxygen. The number of atoms of each are: 2 hydrogen atoms, 1 sulfur atom, 4 oxygen atoms.

Question 2

What is the atomic mass (Ar)?

Answer 2

Top number on the periodic table - sum of protons and neutrons.

Question 3

What is the relative formula mass (Mr)?

Answer 3

Sum of the relative atomic masses of the atoms in the formula (must look at little numbers).

e.g. H2O 2+16 = 18

Question 4

What is the law of conservation of mass?

Answer 4

No atoms are lost or made during a chemical reaction so the mass of the products equals the mass as the reactants.

Question 5

How do the relative formula masses of the reactants compare to the products?

Answer 5

They are equal.

Question 6

Why could the mass of products be less than the mass of the reactants?

Answer 6

A gas is made which can escape e.g. A metal carbonate and acid makes carbon dioxide or during thermal decomposition.

HINT look for (g) state symbols after the arrow.

Question 7

Why could the mass of products be more than the mass of the reactants?

Answer 7

When a gas is added to a reactant e.g. When magnesium reacts with air oxygen is added.

HINT look for (g) state symbols after the arrow.

Question 8

How do you calculate % element in a compound?

Answer 8

% element = (Atomic mass of the element x number of atoms x 100) / Relative formula mass of the compound.

e.g. What is the % of oxygen in H2SO4: % element = (1 + 1 + 32 + 16 + 16 + 16 + 16) * 100 = 65%

Question 9

What is concentration measured in?

Answer 9

Grams per decimetre (g/dm)

Question 10

How do you calculate concentration?

Answer 10

concentration = mass (g) ÷ volume (dm³)

Question 11

What units do mass and volume have to be in when calculating concentration?

Answer 11

Mass must be in grams (g), volume must be in decimetres (dm)

Question 12

How do you convert cm³ to dm³?

Answer 12

Divide by 1000

Question 13

How do you convert kg to g?

Answer 13

Multiply by 1000

Question 14

What does resolution mean?

Answer 14

Smallest change in the equipment (e.g. On a ruler it is 1mm or 0.1cm)

Question 15

What does uncertainty mean?

Answer 15

The interval within which the true value of a quantity can be expected to lie.

Question 16

How do you calculate the uncertainty of results?

Answer 16

Uncertainty = range

Question 17

What are chemical amounts measured in?

Answer 17

Moles, the unit is mol

Question 18

How do the number of particles in one mole compare to one mole of water?

Answer 18

They are the same, a mole is a fixed number of particles.

Question 19

What is the mass of one mole?

Answer 19

The relative formula mass in grams.

Question 20

How many moles are reacted in Mg + 2HCl → MgCl₂ + H₂?

Answer 20

1 mole of magnesium with two moles of hydrochloric acid.

Question 21

What unit is used for mass in chemistry?

Answer 21

Grams, g

Question 22

How can you calculate number of moles?

Answer 22

moles = mass (g) ÷ Relative formula mass, Mr

Question 23

How many moles are there in 500g of NaOH?

Answer 23

Mr (NaOH) = 23 + 16 + 1 = 40

moles = 500 ÷ 40 = 12.5 mol

Question 24

How can you calculate mass?

Answer 24

mass = moles x relative formula mass

Question 25

What is the Mr of a compound when 3 moles has a mass of 126g?

Answer 25

Mr = mass ÷ moles ## Footnote Mr = 126 ÷ 3 = 42

Question 26

How to balance an equation using moles?

Answer 26

Step 1: work out any unknown masses (use conservation of mass rule) Step 2: work out the moles of all substances (you will be given their masses) Step 3: Convert the numbers of moles into whole numbers (these are ratios so must multiply all numbers by same amount)

Question 27

Balance an equation example: MgCO₃ → MgO + CO₂

Answer 27

Step 1: work out any unknown masses Step 2: work out the moles of MgO Step 3: work out the moles of MgCO₃ Step 4: work out the moles of CO₂ Step 5: convert the number of moles into whole number ratios

Question 28

What is the limiting reactant?

Answer 28

The reactant that is completely used up in a reaction.

Question 29

What does it mean if the reactant is said to be in excess?

Answer 29

More than enough present to allow the limiting reactant to get fully used up.

Question 30

How to work out the limiting reactant?

Answer 30

Step 1: Calculate the number of moles for each reactant using the grams in the question. Step 2: Pick ONE of the reactants and using the ratios in the balanced equation, work out what the moles you would need for the OTHER reactant. Step 3: Compare the worked out moles in step 2 for the reactant against actual moles worked out in Step 1.

Question 31

How to work out the reactant mass?

Answer 31

Step 1: Balance the symbol equation. Step 2: Work out the moles for the substance you have the mass of. Step 3: Use the ratio to work out the number of moles of the unknown. Step 4: Calculate the mass of the unknown using moles just calculated.

Question 32

What is Avogadro's number?

Answer 32

Avogadro's constant is 6.02 × 10²³.

Question 33

What is the number of particles in one mole called?

Answer 33

Avogadro's number.

Question 34

How do you calculate the mass of one atom?

Answer 34

Mass of one atom = mass of one mole (Ar) / Avogadro's number.

Question 35

How do you calculate the number of atoms/molecules/particles?

Answer 35

Number of atoms = moles x Avogadro's number.

Question 36

What is percentage yield?

Answer 36

The amount of product obtained from a reaction compared to the theoretical amount.

Question 37

How to calculate percentage yield?

Answer 37

% yield = (mass of product actually made / theoretical mass) x 100.

Question 38

What value is % yield normally between?

Answer 38

0 to 100%.

Question 39

Why is yield below 100%?

Answer 39

Reaction may not go to completion, product may be lost, some reactants may react differently, or side reactions may occur.

Question 40

Why would industry want a high yield?

Answer 40

Reduces waste and cost.

Question 41

What is atom economy?

Answer 41

A measure of the amount of starting materials that end up as useful products.

Question 42

Why is atom economy important?

Answer 42

Sustainable development and economic reasons.

Question 43

Why would industry want a high atom economy?

Answer 43

Less waste, saves resources, increased profit.

Question 44

How to calculate atom economy?

Answer 44

Atom economy = (relative formula mass of desired product / sum of relative formula masses of all reactants) x 100.

Question 45

How can a company increase its atom economy?

Answer 45

By selling off any by-products.

Question 46

What are the units for concentration?

Answer 46

mol/dm³

Question 47

How do you calculate concentration?

Answer 47

Concentration (c) = moles / volume

Question 48

What is concentration mol/dm³ related to?

Answer 48

It relates to the amount of solute in a given volume of solution.

Question 49

How do the volumes of gases compare?

Answer 49

Equal moles occupy the same volume at room temperature and pressure (rtp).

Question 50

What is the volume of one mole of any gas at room temperature and pressure?

Answer 50

24 dm³

Question 51

What is the formula for calculating the volume of a gas from its mass?

Answer 51

Volume of gas = (mass of gas / Mr of gas) x 24

Question 52

What volume of gas does 1.6kg of Bromine gas occupy?

Answer 52

Volume = 1600 ÷ 160 x 24 = 240 dm³ ## Footnote Mr of Br₂ = 160 (2 x 80), Convert 1.60Kg to g = 1600g.