Question

How many grams of chloride ions are in 8.3 g of copper (II) chloride

Answer 1

`4.4\; {\rm g}`.

Step-by-step explanation

Refer to a modern periodic table for relative atomic mass:

• Cu- 63.546;
• Cl- 35.45.

Formula of copper (II) chloride: CuCl₂.

Formula mass of copper (II) chloride:

63.546 + 2 × 35.45 =
`134.446\; {\rm g \cdot mol^(-1)}`.

How many
`{\rm mol}` of formula units in 8.3 grams of CuCl₂?

`(8.3\; {\rm g}) / (134.446\; {\rm g \cdot mol^(-1)}) \approx 0.0617\; {\rm mol}`

Note the subscript "2" in the formula CuCl₂. There are
`2\; {\rm mol}` of chloride ions in every
`1\; {\rm mol}` formula unit of CuCl₂.

There would be
`2 * 0.0617\; {\rm mol} \approx 0.123\; {\rm mol}` of
`{\rm Cl^(-)}` ions in
`0.0617\; {\rm mol}` formula units of CuCl₂.

The mass of
`1\; {\rm mol}` of chloride ion is 35.45 grams.

The mass of
`0.123\; {\rm mol}` of
`{\rm Cl^(-)}` ions will be
`35.45 * 0.123 \approx 4.4\; {\rm g}`.

Round the final value to two sig. fig. since the mass in the question is also 2 sig. fig.