Page 180 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 11] MOLARITY 169

11.15. What is the concentration of a solution prepared by diluting 3.0 L of 2.5 M solution to 8.0 L with water?
Ans. The number of moles of solute is not changed by addition of the water. The number of moles in the original
solution is
2.5 mol

3.0L = 7.5 mol
1L
That 7.5 mol is now dissolved in 8.0 L, and its concentration is
7.5 mol
= 0.94 M
8.0L
11.16. What is the concentration of a solution prepared by diluting 0.500 L of 1.80 M solution to 1.50 L with
solvent?
Ans. The number of moles of solute is not changed by addition of the solvent. The number of moles in the original
solution is
1.80 mol

0.500 L = 0.900 mol
1L
That 0.900 mol is now dissolved in 1.50 L (compare Problem 11.17), and its concentration is
0.900 mol
= 0.600 M
1.50 L
11.17. What is the concentration of a solution prepared by diluting 0.500 L of 1.80 M solution with 1.50 L of
solvent?
Ans. The number of moles of solute is not changed by addition of the solvent; it is 0.900 mol. What is the final
volume of the solution? If we add 1.50 L of solvent, it will be about 2.00 L. Compare this wording with that
of Problem 11.16. The 1.50 mol is now dissolved in 2.00 L, and its concentration is
0.900 mol
= 0.450 M
2.00 L
11.18. What is the concentration of a solution prepared by diluting 25.0 mL of 3.00 M solution to 60.0 mL?

Ans. The number of moles of solute is not changed by addition of the solvent. The number of moles in the original
solution is
(0.0250 L)(3.00 mol/L) = 0.0750 mol
That 0.0750 mol is now dissolved in 0.0600 L, and its concentration is
0.0750 mol
= 1.25 M
0.0600 L
Alternatively, the number of millimoles is given by
(25.0mL)(3.00 mmol/mL) = 75.0 mmol
The concentration is
75.0 mmol
= 1.25 M
60.0mL
The use of millimoles and milliliters saves conversions of milliliters to liters.

11.19. What concentration of salt is obtained by mixing 20.0 mL of 3.0 M salt solution with 30.0 mL of 2.0 M
salt solution?
Ans. The final concentration is the total number of millimoles divided by the total number of milliliters. The
volume is 20.0 mL + 30.0 mL = 50.0 mL. The total number of millimoles is given by

(20.0mL)(3.0 mmol/mL) + (30.0mL)(2.0 mmol/mL) = 120 mmol
The concentration is 120 mmol/50.0 mL = 2.4 M.