Quantitative Aspects of Chemical Change

Question 1

Calculate the molar mass of a substance given its formula

Answer 1

Number of atoms x relative atomic mass

Answer with unit g.mol^-1 (dot is in the middle)

Question 2

Mass, molar mass and number of moles equation

Answer 2

n = m/M

Number of moles = mass/molar mass

Question 3

Determine the empirical formula for a given substance from percentage composition

Answer 3

1. Calculate the number of moles of each element (n = m/M)
   (eg: C was 26,67%. Therefore n(C) = 26,67/12 = 2,22 mol
2. Put the answers for the number of moles in a ratio.
   (eg: 2,22 : 4,44 : 2,22)
3. Then find the LCM of the ratio
   (C:O:H –> 1:2:1)
4. Use that ratio for the elements in the compound
   CO2H = 12 + 2(16) + 1 = 45
5. Molecular formula contains twice as many atoms …
   C2O4H2

Question 4

Determine percentage composition of a compound

Answer 4

1. Work out the molar mass of the compound
2. Take the number of molecules of the element and divide by molar mass
3. Then multiply answer by 100

Question 5

Concentration equation

Answer 5

C = n/V
Concentration = number of mol/volume
(n = C/V)

Question 6

Concentration unit

Answer 6

mol.dm^-3 (dot is in the middle)

Question 7

Determine percentage yield

Answer 7

Percentage yield = actual yield/theoretical yield x 100%

Question 8

Mass (of a mole) equation

Answer 8

n = m/M
Therefore
m = nM

Question 9

Volume (gases and mole equation

Answer 9

n = V/V(m)
number fo moles of gas = volume of gas (dm^3)/ 22.4 dm^3 (it is the STP)
Therefore
V = nV(m)

the ‘m’ is a small letter