Relative Formula Mass

Question 1

What is the relative formula mass of a compound?

Answer 1

The relative formula mass (M,) is the sum of the relative atomic masses of all the atoms in the molecular formula.

Relative atomic masses can be found on the periodic table.

Question 2

How do you find the relative formula mass of MgCl2?

Answer 2

Add the relative atomic mass of magnesium and twice the relative atomic mass of chlorine: Mg + (2 x Cl) = 24 + (2 x 35.5) = 95.

The relative atomic mass of Mg is 24 and Cl is 35.5.

Question 3

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

Answer 3

Percentage mass of an element = (A, x number of atoms of that element / M, of the compound) × 100.

A, is the relative atomic mass of the element.

Question 4

What is the percentage mass of sodium in sodium carbonate (Na2CO3)?

Answer 4

43%.

Calculation: M, of Na2CO3 = (2 x 23) + 12 + (3 x 16) = 106.

Question 5

If a mixture contains 20% iron ions by mass, how much iron is in 50 g of the mixture?

Answer 5

10 g of iron.

This is calculated as 50 g x 20% = 10 g.

Question 6

What is the percentage mass of iron in iron chloride (FeCl2)?

Answer 6

44.09%.

Calculation: Percentage mass = (A x number of atoms of that element x 100) / M, of the compound.

Question 7

How much iron chloride (FeCl2) is needed to provide 10 g of iron?

Answer 7

23 g of iron chloride.

Calculation: If FeCl2 contains 44.09% iron by mass, then to find the mass of FeCl2 that contains 10 g of iron, use the formula: mass = 10 g / 0.4409.

Question 8

Fill in the blank: The relative atomic mass of chlorine (Cl) is ______.

Answer 8

35.5.

Question 9

True or False: The relative formula mass of a compound is the same as the molar mass.

Answer 9

True.

Question 10

What is the relative atomic mass (A,) of sodium (Na)?

Answer 10

This value is derived from the periodic table.