Question

A sample of silver chromate has a mass of 25.8 g:1- Write the formula for silver chromate. 2- Calculate the number of positive ions in it? 3- How many negative ions are there? 4- Calculate the mass in grams of one chemical formula unit?

Answer 1

1. Let's write the silver chromate using the oxidation number of each compound.

Oxygen has a -2 oxidation number, silver only contains +1 and chromate contains +2, 3, 6.

Remember that the ion chromate contains:

`CrO^(2-)_4`

So, in our case we're going to use 2 atoms of Ag to complete the formula, like this:

`Ag_2CrO_4.`

2. Calculating the positive ions, in this case, corresponds to silver and chromate. In the last question, we realize that the ions of silver are +2 and the chromium was 6+. The oxidation number of oxygen is always negative so, this doesn't count. The positive ions are 2*2+6*1= +10 ions.

3. As we see before, oxygen is the only one that contains negative ions, so, we have 4 moles of oxygen which you can see in the formula. Multiplying this number by -2, we obtain -8 ions.

4. To calculate the mass we're going to use the molar mass of each element where you can find on the periodic table. (molar mass of oxygen = 16 g/mol, chromium = 52 g/mol and silver = 107.8 g/mol).

We multiply each molar mass with each mole of each element and then we're going to sum all of them, like this:

`O=16\cdot4=64\text{ g/mol, Cr=52}\cdot1=52\text{ g/mol, Ag=107.8}\cdot2=215.6\text{ g/mol,}`

`64\text{ g/mol + 52 g/mol + 215.6 g/mol =331.6 g/mol.}`

The mass of a chemical formula unit is 331.6 g/mol.