Topic 3 - Quantitive Chemistry

Question 1

What is the formula needed to find the percentage mass of an element in a compound?

Answer 1

( Atomic mass x number of atoms of that element ) divided by the Relative formula mass x 100

Question 2

One mole of a substance is just how much of it that contains the Avogadro constant number of particles. What is the Avogadro constant?

Answer 2

6.02x10(23)

Question 3

What is the formula involving “moles”, “mass” and “relative formula mass” ?

Answer 3

Moles = mass divided by rfm

Question 4

What is the method for writing a balanced equation when it tells you the mass of substances which react and the mass of the products?

Answer 4

• work out the relative formula mass for each part of the equation
• divide the mass of each part by its relative formula mass
• divide the moles by the smallest number of moles
• write the balanced equation

Question 5

What method would you use if you were trying to find the mass of a product formed and it gives you the mass of the limiting reactant?

Answer 5

• write out the balanced equation
• work out the rfm
• calculate the moles of the limiting reactant
• look at the ratio in the question
• calculate the mass using “mass=moles x rfm”

Question 6

What is the formula to find the volume of gas?

Answer 6

Mass divided by rfm x 24

Question 7

What is the formula used to find the concentration of a solution?

Answer 7

mass/moles divided by volume

Question 8

What are titrations?

Answer 8

Experiments which find the volumes of two solutions needed to react together completely.

Question 9

What is atom economy?

Answer 9

How much of the mass of the reactants is wasted and how much ends up as useful product.

Question 10

What’s the formula for finding atom economy?

Answer 10

relative formula mass of desired products divided by relative formula mass of all products x 100

Question 11

What is the formula for finding the percentage yield?

Answer 11

Mass of the product actually made divided by max theoretical mass x 100

Question 12

Suggest some reasons why you can never get a 100% yield.

Answer 12

• reactants always get lost along the way
• in reversible reactions, the reverse reaction is happening at the same time so it never reaches completion
• the reactants could react with gases in the air and impurities in the reaction mix so they form extra products
• when you filter a liquid, you can lose some solid / liquid