Question

Convert 6.35 grams of aluminum sulfate to moles​

Answer 1

There are 0.0186 moles of formula units in 6.35 grams of aluminum sulfate
`\rm Al_2(SO_4)_3`.

Step-by-step explanation:

What's the empirical formula of aluminum sulfate?

Sulfate is an anion with a charge of -2 per ion. When sulfate ions are bonded to metals, the compound is likely ionic.

Aluminum is a group III metal. Its ions tend to carry a charge of +3 per ion.

The empirical formula of an ionic compound shall balance the charge on ions with as few ions as possible.

The least common multiple of 2 and 3 is 6. That is:

• Three sulfate ions
  `\rm {SO_4}^(2-)` will give a charge of -6.
• Two aluminum ions
  `\rm Al^(3+)` will give a charge of +6.

Pairing three
`\rm {SO_4}^(2-)` ions with two
`\rm Al^(3+)` will balance the charge. Hence the empirical formula:
`\rm Al_2(SO_4)_3`.

What's the mass of one mole of aluminum sulfate? In other words, what's the formula mass of
`\rm Al_2(SO_4)_3`?

Refer to a modern periodic table for relative atomic mass data:

• Al: 26.982;
• S: 32.06;
• O: 15.999.

There are

• two Al,
• three S, and
• twelve O

in one formula unit of
`\rm Al_2(SO_4)_3`.

Hence the formula mass of
`\rm Al_2(SO_4)_3`:

`\underbrace{2* 26.982}_(\rm Al) + \underbrace{3* 32.06}_(\rm S) + \underbrace{12* 15.999}_(\rm O) = \rm 342.132\;g\cdot mol^(-1)`.

How many moles of formula units in 6.35 grams of
`\rm Al_2(SO_4)_3`?

`\displaystyle n = (m)/(M) = \rm (6.35\;g)/(342.132\;g\cdot mol^(-1)) = 0.0186\;mol`.