Quantitative Chemistry

Question 1

How do you calculate relative formula mass ( Mr )

Answer 1

work out how many atoms of each element are there in the chemical formula then add the relative atomic masses ( Ar )

Question 2

What is the equation that links no. of moles, mass and rfm (Mr)

Answer 2

No. of moles = Mass (g) / Relative formula mass (Mr)

Use this for compounds

Question 3

What is the equation that links no. of moles, mass and relative atomic mass (Ar)

Answer 3

No. of moles = Mass (g) / Relative Atomic Mass (Ar)

Use this for elements

Question 4

Here is an example of reacting mass question. Understand the ratio method!

Answer 4

Example
12 g of magnesium reacts completely with excess hydrochloric acid to form magnesium chloride and hydrogen:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Calculate the maximum mass of hydrogen that can be produced. (Ar of Mg = 24, Mr of H2 = 2)
Amount of magnesium = Mass/(Relative Atomic Mass)
Amount of magnesium = 12/24
= 0.5 mol
Looking at the equation, 1 mol of Mg forms 1 mol of H2, so 0.5 mol of Mg forms 0.5 mol of H2
Mass of H2 = Mr × amount
= 2 × 0.5
= 1 g

Question 5

How do you work out percentage mass

Answer 5

Total Ar of the element
Percentage Mass= ——————————— * 100
Mr of the Compound

Question 6

What is the theoretical yield of a reaction

Answer 6

result of a rection that is worked out with mole calculations

Question 7

Why in reality is the theoretical yield never produced

Answer 7

• The reaction may not go to completion
• Reactants may be impure
• If it is a gas some could escape
• Some reactions may be reversible
• A side reaction may occur

Question 8

What is the equation for % yield

Answer 8

original mass obtained
% yield = ———————————— * 100
theoretical mass

Question 9

What is percentage atom economy

Answer 9

efficiency of a chemical process in terms of all atoms involved and the desired products produced.

Question 10

What is the equation for percentage atom economy

Answer 10

RFM of Wanted Product
% atom economy = ————————————– * 100
Total RFM of All Products

Question 11

How do you calculate moles in solution using g/dm^3

Answer 11

Mass (g)
Conc. of solutions (g/dm^3) = ————————-
Volume (dm^3)

Question 12

How do you calculate moles in solution using moles/dm^3

Answer 12

Moles
Conc. of solutions (mol/dm^3) = ————————-
Volume (dm^3)

Question 13

What is the conversion between cm^3 , dm^3 and m^3

Answer 13

Divide 1000 divide 1000
Cm3 —————–> dm3—————–>m3
*1000 *1000

Question 14

What is the process of titration

Answer 14

Finding out the concentration of a solution by finding out at what volume it will neutralise after adding a standard (known) solution

Question 15

What is the Law of Conservation of Mass

Answer 15

Key Concept: No atoms are lost or made during a chemical reaction so the:

’ mass of products = mass of the reactants’

Question 16

What are ions

Answer 16

atoms with a charge

Question 17

How do ionic compounds form

Answer 17

when metals react with non metals

Question 18

What do metals and most non metals form

Answer 18

metals form positive ions, non metals form negative ions

Except H+ and NH4+

Question 19

What is charge dependent on

Answer 19

Group number; except transition metals

Question 20

What is Avogadro’s Constant

Answer 20

1 Mole is 6.02x10^23

Question 21

How can Avogadro’s Constant be used

Answer 21

work out the number of atoms in a specific amount of a compound

e.g. Calculate the no. of atoms in 48g of magnesium. Ar of Magnesium= 24

No. of moles = mass/Mr
Moles = 48/24=2 moles
2 moles = 2 * 6.02x10^23 no. of atoms = 1.204x10^24

Question 22

Define a limiting reactant

Answer 22

the reactant that is all used up is called the limiting reactant - it sets a limit on how much the excess product can form.

Question 23

How do you use limiting reactants in calculation

Answer 23

use the limiting reactant to calculate the mass of the product

Question 24

How much volume does 1 mole of any gas occupy

Answer 24

a volume of 24dm3 ( at room temp. and pressure)

room temp. = 20 C Room pressure = 1 atmosphere

Question 25

What is the equation for volume of a gas

Answer 25

Volume (dm^3) = No. of moles * 24