Calculate the molarity of a solution of sodium hydroxide,NaOH, if 23.64 mL of th

Question

Calculate the molarity of a solution of sodium hydroxide,NaOH, if 23.64 mL of this solution is needed to neutralize 0.5632 gof potassium phthalate. Is is found that 24.68 mL of 0.1165 M NaOH is needed totitrate 0.2931 g of an unknown acid to the phenolphthalein endpoint. Calculate the equivalent mass of the acid. Calculate the molarity of a solution of sodium hydroxide,NaOH, if 23.64 mL of this solution is needed to neutralize 0.5632 gof potassium phthalate. Is is found that 24.68 mL of 0.1165 M NaOH is needed totitrate 0.2931 g of an unknown acid to the phenolphthalein endpoint. Calculate the equivalent mass of the acid.

Explanation / Answer

I think potassium phthalate is the same as potassium hydrogenphthalate, in which case its formula isKH(C8H4O4), which is usuallyabbreviated to KHP (the phthalate ion has a charge of 2-, i.e.P2-). I had to look up the reaction on the net; it's acommonly used method of standardising (finding the exactconcentration of) NaOH. The reaction looks like this:     NaOH + KHP --> KNaP +H2O Calculate the moles of KHP (MW = 204.2212 g mol-1):    n(KHP) = 0.5632 / 204.2 g mol-1 =2.758x10-3 mol Since the reaction is 1:1, this means:    n(NaOH) = n(KHP) = 2.758x10-3 mol    [NaOH] = 2.758x10-3 mol / 0.02364 L =0.1167 M (I'm not sure if this next part is correct; I'm not very familiarwith 'equivalent mass'...) For the second part, the 'chemical equation' is not much help!     Acid + NaOH --> ?? + H2O However, we can still calculate the number of moles of NaOHused:     n(NaOH) = 0.02468 * 0.1165 =2.87522x10-3 mol Equivalent mass, if I understand correctly, should be the mass ofacid that is required to react with one mole of OH-. Wereacted a total of 0.2931 g of the acid with2.87522x10-3 mol OH-, so for 1 mol of OH-, we'dneed:     0.2931 g / 2.87522x10-3 = 101.9 g

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