Calculate the Molarity of the Base?

Question

59.20 mL of 0.488 M HCl is needed to neutralize 16.7 mL of a calcium hydroxide solution. Calculate the molarity of the base.

How do i do this? Thanks! (pls explain, don't just answer)

Answer 1

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The reaction that's happening is

2HCl + Ca(OH)2 --> CaCl2 + 2HOH.

The important thing we get out of that reaction is a ratio: it takes 2 HCl to neutralize one Ca(OH)2. Keep that in mind; we'll need it in a minute.

How much HCl do we have? n = CV = (0.488)(0.0592) = 0.0288896 moles of HCl.

Now, because of our 2:1 ratio, the amount of Ca(OH)2 involved only needs to be half that much. 0.0288896mol HCl reacts with 0.0144448mol Ca(OH)2.

If that amount of calcium hydroxide is in 16.7mL of water, what's the concentration?

C = n/V = 0.0144448 / 0.0167 = 0.864958 mol/L Ca(OH)2.

(If they care about significant digits, your answer should only have three, so 0.865.)