Topic 3 - Quantitative chemistry

Question 1

What is the conservation of mass principle?

Answer 1

Conservation of mass:

— No atoms are lost or made during a chemical reaction so the mass of reactants equals the mass of products

Question 2

How do you calculate the % mass of an element in a compound

Answer 2

% mass = (relative atomic mass of element / relative atomic mass of compound) * 100

Question 3

Why might you see a change in mass during or after an experiment?

Answer 3

There might be a change in mass if one of the products is a gas therefore the gas would escape if it is a non-enclosed system which would cause a decrease in mass. The mass could also increase if there is a reaction with a gas in the air.

Question 4

What is the Avogadro constant?

Answer 4

6.02 * 10^23

Question 5

How do you work out number of moles?

Answer 5

Moles = mass / Mr

Question 6

If any number of moles produced is a decimal number, how do you change all of them to simple whole number?

Answer 6

Divide all the number of moles by the smallest amount of moles

Question 7

What is a limiting reactant and what is the effect of a limiting reactant on the amount of product?

Answer 7

Limiting reactant:
— The reactant that is used up in a reaction. The effect on the product is that they are directly proportional so if one halves then so does the other. The limiting reactant limits the amount of product formed.

Question 8

How do you work out the volume of a gas at room temperature?

Answer 8

Volume of gas = (mass of gas / Mr of gas) * 24

note: it is times 24 because one mole of gas occupies 24dm^3 at 20 degrees

Question 9

How do you work out the concentration of a solution in g/dm^3 and mol/dm^3?

Answer 9

concentration = mass (g) / volume (dm^3)

OR

concentration = number of moles (mol) / volume (dm^3)

Question 10

How do you work out the percentage yield of an experiment?

Answer 10

% yield = (mass of product made / maximum theoretical mass of product) * 100

Question 11

What are the three reasons that the % yield can never be 100%?

Answer 11

• – Reaction is reversible
• – Some product is lost
• – There may be unexpected reactions taking place.

Question 12

What is atom economy and how do you work it out?

Answer 12

Atom economy:
— The % of useful product made in a reaction from the reactants

Equation:
— Atom economy = (relative formula mass of desired product / relative mass of all reactants) * 100