- #1

lolbits

- 6

- 0

## Homework Statement

We did an experiment at school. We heated [tex]Na_2SO_4*10H_2O[/tex] so that the water evaportated. We started with 2 gram of the substance, and ended up with 1.8 grams. Unfortunately, not all the water did evaporate because of the equipment we used at school. So the question is, a) how many moles of water evaporated and b) what would the new formula be?

## Homework Equations

mol = m(g) / M_m

## The Attempt at a Solution

a) Moles of water evaporated:

Know that 0.2 grams of water evaporated.

0.2/18 = 0.011 mol H2O evaporated.

That would be about 2 moles for each mol Na2SO4.

b) 2grams [tex]Na_2SO_4*10H_2O[/tex] [tex]\rightarrow[/tex] [tex]Na_2SO_4*xH_2O[/tex]

[tex]M_m[/tex] of [tex]Na_2SO_4*10H_2O[/tex] is [tex]322g/mol[/tex]

[tex]\frac{2g}{322g/mol}[/tex] gives [tex]0.0062 mol[/tex]

[tex]M_m[/tex] of [tex]Na_2SO_4[/tex] = [tex]142 g/mol[/tex]

[tex]\frac{1.8g}{0.0062 mol}=290.32 g/mol[/tex]

[tex]290.32g/mol - 142g/mol = 148g/mol[/tex]

[tex]M_m[/tex] of [tex]H_2O=18[/tex]

[tex]\frac{142 g/mol}{18g/mol} = 8[/tex]

So the new formula would be:

[tex]Na_2SO_4*8H_2O[/tex]

Is this right?