Calculate Relative Formula Mass Flashcards

All 7 cards in this deck

What is the relative formula mass ($M_r$) of a compound?

The sum of the relative atomic masses ($A_r$) of all the atoms in the numbers shown in the formula.

What are the steps to calculate the relative formula mass ($M_r$) of a compound?

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

1. Count the atoms of each element: 2 H and 1 O.
2. Multiply each $A_r$ by its number of atoms: $2 \times 1 = 2$; $1 \times 16 = 16$.
3. Add them: $M_r = 18$

In a balanced equation, how does the total $M_r$ of the reactants compare with the total $M_r$ of the products?

They are equal, when each $M_r$ is counted in the quantities shown by the multipliers.

How do you deduce a missing $A_r$ or $M_r$ from a balanced equation?

e.g. $3\mathrm{H_2} + \mathrm{RO_3} \rightarrow \mathrm{R} + 3\mathrm{H_2O}$, reactants total 150

1. Total $M_r$ of products = total $M_r$ of reactants = 150.
2. Work out the known products: $3 \times M_r(\mathrm{H_2O}) = 3 \times 18 = 54$.
3. Subtract from the total: $A_r(\mathrm{R}) = 150 - 54 = 96$

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

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

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

e.g. Cl in $\mathrm{CH_3Cl}$ ($A_r$: H = 1, C = 12, Cl = 35.5)

1. Find the $M_r$ of the compound: $12 + 3 + 35.5 = 50.5$.
2. Find the total mass of that element: $1 \times 35.5 = 35.5$.
3. Divide and $\times 100$: $\dfrac{35.5}{50.5} \times 100 = 70.3\%$

True or false? Relative formula mass ($M_r$) has the unit grams.

False. $M_r$ is a relative value and has no units.