Question #533df

1 Answer
Oct 8, 2015

Answer:

#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: