Chemistry help...calculating mass??

Question

What mass of magnesium ions would be available from a packet of epsom salts that contained 375g of hydrated magnesium sulfate?
MgSO4.7H20 <---- the formula of hydrated magnesium sulfate.

Answer 1

This is a very easy and standard question:

First find out how much 1 mole of MgSO4.7H20 weighs

Mg = 24.30 g/mol x 1mol = 24.30 g
S = 32.07 g/mol x 1 mol = 32.07 g
11 x O = 16 g/mol x 11 mol = 176 g
14 x H = 1 g/mol x 14 mol = 14 g

Add them up.... 24.30 + 32.07 + 176 + 14
= 246.1 g

now....

the fraction of Mg is 24.03/246.1 = 0.09764 = 9.764 %

so in 375 g of MgSO4.7H20

there are 9.764 % x 375 g = 36.6 g of Mg

Answer 2

First, get the MW of hydrated magnesium sulfate (246.51g/mol) and atomic weight of Mg (24.31 g/mol).

Get the % by weight (or mass) of Mg in MgSO4.7H2O by:
% w (Mg) = 24.31g/mol / 246.51g/mol =9.86%.

This means 9.86% of the weight of MgSO4.7H2O is Mg. Since you have 375g sample, multiply this by the % w (Mg) to get 36.98g.

Answer 3

if you have 375g of the compound MgSO4.7H20 (molecular weight=246.4) then you have 1.52 moles of hydrated magnesium sulfate.

n(MgSO4.7H20)=m/Mr
=375/246.4
=1.52

and because you can only get one ion of Mg from each molecule of hydrated magnesium sulfate then the mass of Mg ions you can get is 36.99g

m(Mg)=n(Mg)*Mr(Mg)
=1.52*24.3
=36.99g

Answer 4

Density=Mass/quantity. So, once you re-set up the equation, you get: Mass=Density*quantity. which equals 0.798 g/mL * 17.4 mL (here, mL gets cancelled off) that's finally 13.8852 g.