C3 - Relative Formula Mass

What does relative atomic mass (Ar) mean?

The average mass of the isotopes of an element, weighted for the abundance of each isotope.

What does abundance mean in chemistry?

How common an isotope is.

What is an isotope?

Atoms of the same element with the same number of protons but a different number of neutrons.

What is the mass number of chlorine-35?

35 (17 protons + 18 neutrons).

What is the mass number of chlorine-37?

37 (17 protons + 20 neutrons).

Why is chlorine's Ar shown as 35.5 on the periodic table?

Because it is a weighted average of chlorine-35 and chlorine-37, taking into account abundance.

What is the definition of relative formula mass (Mr)?

The sum of the relative atomic masses of all the atoms in the numbers shown in the formula.

What is the symbol for relative formula mass?

Mr.

Does relative formula mass have units?

No, Mr has no units.

Do we use big numbers in front of a formula when calculating Mr?

No, big numbers in front of a formula are not used in Mr calculations.

Example: What is the formula of methane?

CH₄.

What is the relative formula mass of methane (CH₄)?

16 (C = 12, H = 1×4, total = 12+4).

If the formula shows 2CH₄, what is the Mr?

Still 16, because big numbers in front are not used in Mr.

Example: What is the formula of calcium sulfate?

CaSO₄.

What is the relative formula mass of calcium sulfate (CaSO₄)?

136 (Ca = 40, S = 32, O = 16×4 = 64, total = 40+32+64).

Example: What is the formula of magnesium hydroxide?

Mg(OH)₂.

What is the relative formula mass of magnesium hydroxide (Mg(OH)₂)?

58 (Mg = 24, O = 16×2 = 32, H = 1×2 = 2, total = 24+32+2).

Key rule to remember when calculating Mr

Multiply each atom's Ar by how many atoms are present, then add them all together.

Exam tip: What are you always given in an exam to calculate Mr?

The relative atomic masses (Ar) you need.