Question

Question #533df

Answer 1

`MgSO_(4).7H_2O`

Explanation:

The hydrated magnesium sulfate contains water of crystallisation which is driven off when heated:

`MgSO_(4).xH_2O_((s))rarrMgSO_(4(s))+xH_2O_((g))`

By recording the loss in mass we can find the mass of water and then convert masses into moles to find `x`.

Mass `MgSO_(4).xH_2O=4.312"g"`

Mass `MgSO_(4)=2.107"g"`

So mass water lost = `4.312-2.107=2.205"g"`

Now we need to convert grams into moles using:

Number of moles = mass in grams/mass of 1 mole

Or:

`n=m/M_r " "color(red)((1))`

`M_r` is the relative molecular mass for a molecular compound or the relative formula mass for an ionic compound.

We get this by adding all the relative atomic masses (`A_r`) together as they occur in the formula. You should always be provided with these in the question or on a data sheet.

Using accurate `A_r` values we get:

`M_r[MgSO_4]=120.37`

`M_r[H_2O]=18.01`

You should use the `A_r's` you are given - they will be close.

Now using `color(red)((1))rArr`

`nMgSO_4=2.107/120.37=0.0175`

and:

`nH_2O=2.205/18.01=0.12243`

So we now know that the ratio in moles of:

`MgSO_4:H_2O` is:

`0.0175:0.12243`

We like to express this ratio in whole numbers according to how the atoms/molecules combine so we can divide both numbers by `0.0175rArr`

`0.0175/0.0175:0.12243/0.0175rArr`

`1:6.996`

Which is good enough (in my book) as:

`1:7`

So `x=7` which gives the formula for hydrated magnesium sulfate to be:

`MgSO_(4).7H_2O`

Here is a full lab on hydrate and how to calculate the formula of a hydrate, I hope this helps more: