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Question
Calculate [H+], [H2PO4], [HPO2−4] and [PO3−4] in a 0.01M solution of H3PO4.
Take K1=7.225×10−3, K2=6.8×10−8, K3=4.5×10−13.
Calculate [H+], [H2PO4], [HPO2−4] and [PO3−4] in a 0.01M solution of H3PO4.
Take K1=7.225×10−3, K2=6.8×10−8, K3=4.5×10−13.
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Solution
The correct option is C [H+]=[H2PO−4]=5.6×10−3, [HPO2−4]=6.8×10−8, [PO3−4]=5.4×10−18
(i) H3PO4⟶H++H2PO−4; K1=7.225×10−3
0.01M C (1−α) Cα1 Cα1
(ii) H2PO−4⟶HPO2−4+H+ K2=6.8×10−8
Cα(1−α2) Cα1α2 [Cα1]
(iii) HPO2−4⟶PO4−3+H+ K3=6.8×10−8
α1α2C1(1−α3) C(α1α2α2)[Cα1]
7.225×10−3=Cα21(1−α1)=0.01×α211−α
R×N. (i)
(1−α)×0.7225=α21
α21+0.7225α−0.7225=0
α1=0.562
⇒ [H+]=0.01×0.562 [H+=5.6×10−3
[H2PO⊖4]≈5.6×10−3 R×N. (ii)
6.8×10−8=[HPO2−4][H+][H2PO−4] from (i) reaction.
[HPO2−4=6.8×10−8M R×N (iii)
4.5×10−13=[PO−34][H+][HPO2−4]
4.5×10−13×6.8×10−55.6×10−3=[PO3−4]
5.464×10−18=[PO−34]
(i) H3PO4⟶H++H2PO−4; K1=7.225×10−3
0.01M C (1−α) Cα1 Cα1
(ii) H2PO−4⟶HPO2−4+H+ K2=6.8×10−8
Cα(1−α2) Cα1α2 [Cα1]
(iii) HPO2−4⟶PO4−3+H+ K3=6.8×10−8
α1α2C1(1−α3) C(α1α2α2)[Cα1]
7.225×10−3=Cα21(1−α1)=0.01×α211−α
R×N. (i)
(1−α)×0.7225=α21
α21+0.7225α−0.7225=0
α1=0.562
⇒ [H+]=0.01×0.562 [H+=5.6×10−3
[H2PO⊖4]≈5.6×10−3 R×N. (ii)
6.8×10−8=[HPO2−4][H+][H2PO−4] from (i) reaction.
[HPO2−4=6.8×10−8M R×N (iii)
4.5×10−13=[PO−34][H+][HPO2−4]
4.5×10−13×6.8×10−55.6×10−3=[PO3−4]
5.464×10−18=[PO−34]
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