Calculations

Question 1

What is molecular mass?

Answer 1

The total relative atomic mass in a compound

Question 2

How many particles are in a mole?

Answer 2

6.023x10²³

Question 3

What is the name for this number?

6.023x10²³

Answer 3

Avogadro’s Constant

Question 4

How do you calculate Mr when given the mass and moles?

Answer 4

. mass
Mr = —————
moles

Question 5

What is empirical formula?

Answer 5

The simplest whole number ratio of atoms in a compound

Question 6

If given the masses of the different elements in a compound, how would you find the empirical formula?

Answer 6

• Calculate the moles in each element
  moles = given mass / Mr
• Divide each number of moles by the smallest number of moles to get the ratio of atoms
• Simplify this into the empirical formula

Question 7

The empirical formula of a compound is CH3. The Mr is 45. What is the molecular formula?

Atomic mass of carbon: 12
Atomic mass of hydrogen: 1

Answer 7

• Calculate the Mr of the empirical formula
  12 + (1x3) = 15
• Divide the Mr of the molecular formula by the Mr of the empirical formula
  45/15 = 3
• Multiply the empirical formula by the number calculated in Step 2
  CH3 x 3 = C3H9

Question 8

How do you find the atom economy of a product in a reaction?

Answer 8

. Mr of useful product
Atom economy = ——————————- x100
Total Mr of reactants

Question 9

What is atom economy?

Answer 9

The percentage, by mass, of all useful products formed in a reaction

Question 10

How would you calculate percentage yield?

Answer 10

. Actual yield
Percentage yield = ———————— x100
Theoretical yield

Question 11

What is theoretical yield?

Answer 11

The maximum mass of product that can be formed from a given amount of reactant

Question 12

How would you calculate the concentration of a solution?

Answer 12

. Amount of substance
Concentration = ——————————-
Volume

Question 13

How could you calculate the percentage by mass of an element in a compound?

Answer 13

. Mass of element
Percentage by mass = ————————- x100
Mr of compound

Question 14

What is molar volume?

Answer 14

The volume occupied by one mole of any gas

Question 15

How do you calculate the volume of a gas using molar gas volume?

Answer 15

Moles of gas
——————
24

Question 16

How do you calculate Mr when given the relative atomic masses of each element?

Answer 16

Add the relative atomic masses of each atom

Question 17

What is the symbol for relative atomic mass?

Answer 17

A_r

Question 18

Describe an experiment to determine the empirical formula of a compound

Answer 18

• Record the mass of an empty crucible and lid
• Put a magnesium ribbon in and record the mass again
• Heat the crucible over a Bunsen burner until the magnesium stops glowing
• Record the mass of the crucible and contents again

Question 19

What is the law of conservation of mass?

Answer 19

Atoms cannot be created or destroyed

Question 20

48g Mg + ??? O ——> 80g MgO

Answer 20

80 - 48 = 32

32g

Question 21

How do you calculate the number of atoms in one mole of a compound?

Answer 21

Number of atoms in each compound x Avogadro’s number

Question 22

How do you calculate the number of particles of a substance?

Answer 22

Number of moles of the substance x Avogadro’s number

Question 23

What causes a reaction to stop happening?

Answer 23

One of the reactants being used up