Multiply the volume expressed in liters times the molar concentration times the
gram molecular weight to determine the amount of hydrate salt, in grams, needed to
prepare the solution.
2.00 mol
249.6 g
0.250 L X -------- = ---------- = 125 g
L
mol
3-10. PROBLEMS INVOLVING ONLY PART OF THE MOLECULE
When preparing solutions using compounds formed by ionic bonding, realize that
these molecules will ionize when in solution. For example, one mole of Na2SO4 will
ionize to two moles of sodium and one mole of sulfate radical. If we were to prepare a
one mol/L sodium solution using Na2SO4, then only 0.5 moles of sodium sulfate would
be needed for every liter of solution since two moles of sodium are contained in one
mole of sodium sulfate. When solving this type of problem use an appropriate factor to
account for the moles of desired substance per mole of compound.
Example. How much Na2SO4 is needed to prepare 2.00 liters of 1.50 mol/L
sodium solution?
Solution. Read the problem carefully and determine the desired quantity.
Grams of Na2SO4.
Calculate the GMW of the compound.
Na2SO4
Na
23.0 X 2 = 46.0
S
32.1 X 1 = 32.1
O
16.0 X 4 = + 64.0
142.1 g/mol
Multiply the volume expressed in liters times the molar concentration times the
gram molecular weight times the ratio of ionized sodium per mole of compound to
determine the amount of salt, in grams, needed to prepare the solution.
1.50 mol Na
142.1 g
1 mol Na2SO4
2.00 L X ------------ X -------- X ---------------- = 213 g Na2SO4
L
mol
2 mol Na
MD0837
3-10