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4.0 g of sodium hydroxide NaOH was dissolved in sufficient water to make up 125 mL of solution.

A) Find the concentration of the solution.

B) What mass of sodium hydroxide could be obtained by heating a 20 mL sample of this solution to dryness?

C) If 30 mL of the solution were mixed with sufficient water to produce 480 mL of a more dilute solution, what would be the concentration of the dilute solution?

D) If 200 mL of the more dilute solution were then mixed with 100 mL of the more concentrated solution, what would be the concentration of the mixed solution?

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Answer:

(A) 0.80 M

(B) 0.64 g

(C) 0.050 M

(D) 1.6 M

Explanation:

(A)

Convert 4.0 g to moles with molar mass. Convert 125 mL to L.

4.0 g x (1 mol/39.998 g) = 0.100 mol

125 mL x (1 L/1000 mL) = 0.125 L

Molarity = mol/L

Molarity = 0.100 mol/0.125 L = 0.80 M

(B)

Find moles of NaOH with the molarity you just found. And convert 20 mL to L.

Molarity = mol/L

0.80 M = mol/0.020 L

mol = 0.016

Convert moles to grams with molar mass.

0.016 mol x (39.998 g/1 mol) = 0.64 g

(C)

You know the molarity of a NaOH solution (Part A). 30 mL initial volume and 480 mL final volume are given. Dilution question so use the dilution formula.

M1V1 = M2V2

(0.80 M)(30 mL) = (M2)(480 mL)

M2 = 0.050 M

(D)

Dilution question again, use the dilution formula.

M1V1 = M2V2

(0.80 M)(200 mL) = (M2)(100 mL)

M2 = 1.6 M

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