Relative Formula Mass Flashcards

All 6 cards in this deck

What is relative formula mass ($M_r$)?

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

What are the steps to calculate a relative formula mass?

e.g. $\mathrm{CaCO_3}$ ($A_r$: C = 12, O = 16, Ca = 40)

1. Multiply each $A_r$ by the number of those atoms: $40$, $12$, $3 \times 16 = 48$
2. Add them: $40 + 12 + 48 = 100$

What is the formula for the percentage by mass of an element in a compound?

$\%$ by mass $= \dfrac{A_r \times \text{number of atoms of that element}}{M_r \text{ of compound}} \times 100$

True or false? Relative formula mass has the unit grams.

False. Relative formula mass is a ratio and has no units.

What are the steps to find the percentage by mass of an element in a compound?

e.g. % of O in $\mathrm{H_2O}$ ($A_r$: H = 1, O = 16)

1. Find $M_r$: $2 + 16 = 18$
2. Find total mass of that element: $16$
3. Divide and $\times 100$: $\frac{16}{18} \times 100 = 88.9\%$

True or false? In $\mathrm{POCl_3}$ the mass of chlorine used in a percentage-by-mass calculation is $3 \times 35.5$.

True. You must multiply the $A_r$ by the number of atoms of that element in the formula.