Calculations

Question 1

Theoretical yield

Answer 1

% yield = Actual yield x 100 Theoretical yield

Question 2

Reasons why you didn’t collect the full theoretical yield

Answer 2

maybe the reaction wasn’t finished?

• maybe some of the gas escaped acci- dentally.
• maybe the hydrogen reacted with something else before you could collect. it?

Question 3

Calculating volume from masses

Answer 3

1) Note ratio 2NH4Cl : 2NH3 (ie 1:1)
2) Mr of known reactant NH4Cl = 53.5
3) work out no. of moles (n) on known reac- tant:n=m÷Mr=10÷53.5=0.187mol
4) Remember 1:1 ratio! So 0.187 moles of NH4Cl will make 0.187 moles of NH3.
5)v=nxVm=nx24=0.187x24
v=4.49dm3

Question 4

Titrations

Answer 4

NaOH
v = 25cm3
v = 0.025dm3
c = 0.50mol dm-3 n = c x v
= 0.50 x 0.025 = 0.0125 mol
HCl
v = 22.85cm3
v = 0.02285dm3
c = n ÷ v
= 0.0125 ÷ 0.02285
-3 = 0.547 mol dm

Question 5

Number moles =

Answer 5

Concentration x volume

Question 6

Mass

Answer 6

No. of moles x relative mass

Mr = CaCl2 = 40 + (35.5 x 2) = 111
n = m ÷ Mr = 11.1 ÷ 111 = 0.1 c = n ÷ v = 0.1 ÷ 0.5
= 0.2 mol dm-3

Question 7

Cm3 into dm3

Answer 7

= divide by 100

Question 8

Concentration =

Answer 8

Mass/volume

Question 9

Atom economy

Answer 9

relative mass of useful product/relative mass of all products x 100

Question 10

Which reaction pathway should I choose?

Answer 10

least by-products, so high Atom Economy - Or find uses for my by products.
- High rate of reaction but low energy input needed to do it.

Question 11

Molar volume =

Answer 11

Volume/no. Of moles

Question 12

1 mole of gas will occupy

Answer 12

A volume of 24dm3 at room temp and pressure

Question 13

How to determine empirical formula of a single compound

Answer 13

1. Weigh crucible and lid
2. Weigh crucible, lid and clean magnesium
3. Heat crucible strongly, occasionally lifting the lid to let oxygen in
4. Reweigh the crucible, lid and magnesium oxide and heat again
5. Reweigh and if mass stays the same reaction is complete
   Do final mass - Nass of crucible, lid and magnesium to give you mass of oxygen reacted

Question 14

Relative formula mass

Answer 14

Sum of all relative atomic masses

H2O = (2x1) + 16 = 18

Question 15

Empirical formula

Answer 15

Mass
R.A.M
Moles (mass/r.a.m)
/ smallest
Formula

Question 16

Conservation of mass

Answer 16

The total mass of reactants always equals the total mass of products (in a closed system)

Question 17

1 mole

Answer 17

6.02x10^23

1 mole = relative mass in grams

Question 18

Avagardo’s number

Answer 18

No. of particles / no. of moles = 6.02x10^23