Help with Chemistry problem, thank you?

2017-10-30 12:12 pm

Gifblaar is a small South African shrub and one of the most poisonous plants known because it contains fluoroacetic acid (FCH2COOH), which has a pKa of 2.59. Calculate the concentration of fluoroacetic acid in a solution that has the same pH as a 0.00210 M solution of HCl.

I keep getting .00172, but that is incorrect, thanks for helping :)

回答 (1)

micatkie

2017-10-30 2:02 pm

pKa = -logKa = 2.59
Hence, Ka = 10⁻²·⁵⁹

Let y M be the concentration of FCH₂CH₂COOH.

HCl is a strong acid that completely dissociates to give H⁺ ions.
The FCH₂CH₂COOH has the same pH as a 0.00210 M HCl.
Hence, [H⁺] in FCH₂CH₂COOH solution = 0.00210 M

Consider the dissociation of FCH₂CH₂COOH :
_______ FCH₂CH₂COOH(aq) ⇌ FCH₂CH₂COO⁻(aq) + H⁺(aq) ___ Ka = 10⁻²·⁵⁹
Initial: _______ y M ___________ 0 M __________ 0 M
Change: ___ -0.00210 M ____ +0.00210 M ___ + 0.00210 M
At eqm: _ (y - 0.00210) M ____ 0.00210 M ____ 0.00210 M

At equilibrium :
Ka = [FCH₂CH₂COO⁻] [H⁺] / [FCH₂CH₂COOH]
10⁻²·⁵⁹ = 0.00210² / (y - 0.00210)
y = (0.00210²/10⁻²·⁵⁹) + 0.00210
y = 0.00382

Concentration of FCH₂CH₂COOH = 0.00382 M

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