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

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

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