Calculations

Question 1

how would you calculate the formula mass of a substance

ie H2SO4

Answer 1

find the mass number of each element in the substance
(ie hydrogen is 1)

and multiply the number if the element is used multiple times
(ie hydrogen is H2 so multiply 2x1)

do the same to the other elements
(S = 32) (O = (16x4)

add the resulting numbers together
(2 + 32 + 64 = 98)

Question 2

how would you calculate the percentage by mass of an element within a substance?
ie Al in Al2O3

Answer 2

calculate the Mr of the element
(ie Al = 27x2)
and the Mr of the substance
(ie Al2O3 = 54+48)

put the element’s Mr over the substance’s Mr
(54/102)

and multiple the result by 100
(54/102 x 100 = 52.9%)

Question 3

how many particles of any substance are in a mole?

Answer 3

6.02 x 10^23

Question 4

what is the name of the number of particles in a mole?

Answer 4

Avogrado’s number

Question 5

one mole is equal to what?

Answer 5

the Ar/Mr of a substance expressed in grams

ie 1 mole of Carbon = 12g

Question 6

what is the Ar of an element?

Answer 6

its mass number

ie oxygen’s Ar = 16

Question 7

what is the equation linking number of moles with mass?

Answer 7

no. of moles = mass/Ar OR Mr

MASS MUST BE IN GRAMS

Question 8

how would you work out the empirical formula of a substance?

3 steps

Answer 8

1. begin with the number of grams or % composition of the elements

(ie 9g of aluminium and 35.5g of chlorine)

2. divide by the Ar to work out the number of moles

(ie Al 9/27 = 0.333 moles AND Cl 35.5/35.5 = 1 mole)

3. divide each answer by the smallest value to give the simplest whole number ratio

(ie Al 0.333/0.333 = 1 AND Cl 1/0.333 = 3

so the empirical formula is AlCl3)

Question 9

how would you work out the molecular formula of a substance (using the empirical formula and the Mr)

Answer 9

1) calculate the Mr of the empirical formula

(ie AlCl3 = 27 + (3x35.5)
= 133.5)

2) divide the Mr of the compound by the Mr of the empirical formula

(ie 267/133.5 = 2

if the answer is 2 multiply the empirical formula by 2
AlCl3 x 2 = Al2Cl3)

Question 10

what tells us the ratio or reacting moles?

Answer 10

balanced equations

Question 11

what are the three steps to work out mass of a substance when given the mass of an element?

(ie 10g of magnesium is burned to produce magnesium oxide. calculate the mass of magnesium oxide)

Answer 11

MOLES
MOLES
MASS

no. of moles of the substance: moles = mass/Mr
use the balanced equation to find the no. of moles of the unknown
calculate the mass using moles x Mr

(ie moles of Mg = 10/24 = 0.4166….
moles of MgO = 0.4166….
mass if Mgo = 0.4166…. x 10 = 16.7g)

Question 12

if i add 2 moles of CaO + 3 moles of H2O to make Ca(OH)2 which substance is in excess and how many moles of the product will i make?

Answer 12

1) H2O (3 moles) is in excess

2) 2 moles will be made

Question 13

if i add 2 mol of 2Ca + 2 mol of O2 to make 2CaO which element is in excess and how many moles of the product will i make?

Answer 13

1) O2 (2 moles) in in excess

2) 4 moles of CaO will be made

Question 14

what is important to remember when calculating with gas?

Answer 14

equal volumes of gas at the same temperature and pressure contain equal numbers if molecules

Question 15

how much space will 1 mole of any gas occupy at room temperature and pressure?

Answer 15

24dm^3 OR 24,000cm^3

1dm^3 = 1 litre = 1000cm^3

Question 16

how would you find the number of moles of 48.0dm^3 of CO at rtp

Answer 16

no of moles = 48 (volume) / 24 (1 mole)

= 2 moles

Question 17

how would you use MOLES MOLES MASS with gases?

ie the volume of CO2 at rtp formed when 6kg of ethane burns in oxygen

C2H6 + 7O2 -> 4CO2 + 6H2O

Answer 17

the same as a regular moles moles mass calculation, but multiplying by 24dm^3 instead of the mass at the end to find the volume

ie moles of ethane = 6000 (mass) / 30 (Mr) = 200
moles of CO2 = 200 x 4 (balanced equation) = 800
volume of CO2 = 800 x 24
= 19200dm^3