Chemical formulae, equations and calculations

Question 1

Relative Formula Mass

Answer 1

The sum of the relative atomic masses of the atoms

Question 2

Relative Atomic Mass

Answer 2

The average mass of its atoms, compared to 1/12th of the mass of a carbon-12 atom

Question 3

State the definition of a mole

Answer 3

A mole is the unit of measurement for an amount of substance

Question 4

State the percentage yield calculation

Answer 4

(Actual yield / theoretical yield) multiplied by 100

Question 5

State the definition of a theoretical yield

Answer 5

The maximum possible mass of a product that can be made in a chemical reaction

Question 6

State the definition of an actual yield

Answer 6

The mass of the product actually obtained from the reaction

Question 7

State the definition of the empirical formula

Answer 7

The simplest whole ratio of atoms of each element in the compound

Question 8

How do you calculate the empirical formula?

Answer 8

1) Write the element symbols
2) Write the masses
3) Write the Ar values
4) Divide masses by Ar
5) Divide by the smallest number
6) Write the formula

Question 9

How do you find the empirical formula for metal oxides?

Answer 9

1) Weigh a crucible (with its lid).
2) Put a sample of clean magnesium ribbon into the crucible and weigh it with the lid. Calculate the mass of magnesium by subtracting the mass of the empty crucible.
3) Strongly heat the crucible over a Bunsen burner for several minutes.
4) Carefully lift the lid from time to time to allow sufficient air into the crucible for the magnesium to fully oxidise without letting any magnesium oxide escape.
5) Continue heating until the mass of the crucible reaches a constant (maximum) mass, indicating that the reaction is complete.
6) Measure the mass of the crucible and contents again. Calculate the mass of the magnesium oxide by subtracting the mass of the empty crucible.

Question 10

How do you find the molecular formula experimentally?

Answer 10

1) Record the mass of an evaporating basin.
2) Add a known mass of hydrated copper sulfate.
3) Heat over a Bunsen burner, gently stirring, until the blue copper sulfate has turned completely white and the mass has reached a constant (minimum) mass, indicating that all the water has been lost.
4) Record the mass of the evaporating basin and contents and then use this to calculate the mass of the white anhydrous copper sulfate remaining, and also the mass of the water lost.

Question 11

What is Avogadro’s Constant?

Answer 11

6.02 x 10^23

Question 12

Amount of moles

Answer 12

Volume/24

Question 13

Concentration equation (moles)

Answer 13

Concentration = Moles/Volume

Question 14

Mass equation (moles)

Answer 14

Mass = Mr x Moles