Consider a solution containing 0.100 M fluoride ions and 0.126 M hydrogen fluoride. The concentration of hydrogen fluoride after addition of 5.00 mL of 0.0100 M HCl to 25.0 mL of this solution is __ M.
A) 0.00976
B) 0.00193
C) 0.100
D) 0.126
E) 0.107

Question

Quizplus · Accepted Answer

The addition of HCl will convert some fluoride ions to hydrogen fluoride. The moles of HCl added are 5.00 mL * 0.0100 M = 0.000050 moles. The initial moles of fluoride ions are 0.100 M * 0.025 L = 0.0025 moles. After reaction, the moles of fluoride ions become 0.0025 - 0.000050 = 0.00245 moles, and the moles of hydrogen fluoride become 0.126 M * 0.025 L + 0.000050 = 0.00315 moles. The total volume is 30.0 mL or 0.030 L, so the concentration of hydrogen fluoride is 0.00315 moles / 0.030 L = 0.105 M, which is closest to 0.107 M.

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