quantitative chemistry

Question 1

how do you calculate the percentage mass of one specific element in a compound

Answer 1

atomic mass x number of atoms
——————————————— x100
relative formula mass of compound

Question 2

what is relative formula mass and how might it be written

Answer 2

sum of all atomic masses in a compound
Mr

Question 3

what is avogadros constant (specific number)

Answer 3

6.02 x 10^23

Question 4

what is a mole

Answer 4

the amount of a substance that contains 6.02 x 10^23 particles
(an avogadro number of particles)

Question 5

relationship between moles and relative formula mass

Answer 5

mass (in grams) of one mole of substance/atoms = relative formula mass of the substance/atoms

e.g carbon Ar is 12
1 mol of carbon = 12g

Question 6

formula triangle for mass, relative formula mass, ad number of moles

Answer 6

Question 7

why might mass change in a chemical reaction?

Answer 7

Increase: one of the reactants is a gas and was not accounted in reaction vessel for reactants (e.g metal + oxygen —> metal oxide)
decrease: one of the products is a gas & it escaped

Question 8

How do you workout the balanced equation only by knowing the masses of the reactants and products?

Answer 8

• work out Mr of each substance
• divide mass by Mr for each substance to find moles
• divide each by smallest number of moles in reaction
• write balanced equation with these whole big numbers

Question 9

What is the limiting reactant?

Answer 9

The reactant that gets used up, same volume of product

Question 10

How do you calculate mass of a product only using the mass of the limiting reactant and the balanced equation

Answer 10

• write out the equation
• calculate Mr of product and reactant you want
• calculate moles of the limiting reactant (mass/Mr)
• Look at ratio in equation, rearrange to substitute moles

Question 11

how do you workout the volume of a gas?

Answer 11

Moles of gas x 24

Question 12

what is atom economy?

Answer 12

Percentage of reactants forming useful product - the higher the “greener”

Question 13

what does 100% yield entail?

Answer 13

You got all the products you expect to get - never occurs

Question 14

Why is yield never 100%?

Answer 14

not all reactants react
There might be side reactions
You lose product when you try to separate it from the mixture

Question 15

why is aluminium oil mixed with cryolite

Answer 15

lowers melting point - reduces energy needed and saves money

Question 16

half equations for electrolysis of aluminium oxide

Answer 16

al3+ + 3e- —> al (reduction)
o2- —> o + 2e- (oxidation)
/
2o2- —> o2 + 4e-