How Much Epsom Salt for a 15% Magnesium Sulfate Solution?

[HarryA]

I would like to make a solution of magnesium sulfate (MgSO4) that is 15%
magnesium sulfate by mass in water. As magnesium sulfate is sold as epsom salts which is
MgSO4 7H2O I need a way to calculated the epsom salts required for the required
magnesium sulfate or 1.24 pounds per gallon of water (Water at 8.35 pounds per gallon).I am thinking one goes by atomic weight?
For MgSO4:
Mg= 24.3 S= 32.06 Oxgen4= 4* 16 = 64 MgSO4= 120.36

For 7H2O:
7H2 = 7*1*2 = 14 7Oxgen = 7* 16 = 112 7H2O= 126.0

For MgSO4 7H2O= 120.36 + 126.0 = 246.36

So to get 1.24 pounds of MgSO4: 1.24 * 246.36/120.36 = 2.05 pounds perhaps?

Thanks in advance.

 

[Bystander]

MgSO4= 120.36

Check.

7H2O= 126.0

Check.

MgSO4 7H2O= 120.36 + 126.0 = 246.36

Check.

2.05 pounds perhaps?

Not check.
Take another swing at the proportion. You'll also want to account for the mass of water you're adding as hydrate.

 

[HarryA]

Thanks.
So the ratio of water in epsom salts is: 126.0/246.36 = 51.14 % by weight and
48.86% MgSO4. So 48.86% of what weight of epsom salts is 1.24 pounds of MgSO4?
Epsom salts = 1.24/0.4886 = 2.54 pounds ?

I need to fill my RC blimp envelope with hydrogen.