# What is the amount (in moles) of copper (II) sulfate pentahydrate that you will need to make 100mL of a 0.500 mol/L solution, with all the calculations?

##### 1 Answer

Dissolve 0.05 moles/12.48 g of the pentahydrate salt in the minimum amount of water, then make the solution up to a total volume of 100 mL.

#### Explanation:

You need to take the waters of hydration into account but you still need this many moles of

In terms of number of moles of

Now, you will need the molar mass of the hydrate salt:

And so the mass you need to weigh out is:

This gives you the required number of moles of

So...

You need to dissolve this amount of solid in the minimum amount of water (less than than the final volume) first. Then make the **total** volume up to 100 mL. The waters of hydration have contributed (4.5 g) to the volume of the final solution, meaning that you didn't have to add the full 100 mL. Approximating the density of water to 1 g/mL, you should only have added

To see how I got 4.5 g of water from the hydrate, see below.

If you break the chemical formula down you get one mole of

And multiplying by the molar masses of each component you get:

This is the same mass that we weighed out before, as it should be.