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Which of the following compounds, when added to an aqueous solution of HF, would cause an increase in the pH?
Which of the following compounds, when added to an aqueous solution of HF, would
cause an increase in the pH? Which would cause a decrease in pH? Which would not have an effect on the pH?
NaF
SnF2
HCl
NaCl
NaOH
H2O
My answer (see if it is correct):
NaF, SnF2, NaOH will increase pH. Reason: When NaF or SnF2 is added, then, due to common ion, F^-, equilibrium will shift to left, so H^+ decrease and that means pH will increase. When NaOH is added, then OH^- ion will reduce H^+ concentration, so pH will increase.
HCl will decrease pH. Reason: HCl will add H+, so pH will decrease.
NaCl and H2O will not affect pH.
2 Answers
- pisgahchemistLv 73 years ago
HF and pH
HF is a weak acid (essentially because F is highly electronegative and "hangs onto the hydrogen.")
HF <==> H+ + F- ................ Ka = 7.2x10^-4
An increase in pH occurs if the solution becomes less acidic, that is, the H+ ion concentration decreases. According to Le Chatelier's principle, increasing the fluoride ion concentration will shift the equilibrium to the left and remove some H+ ions from solution (forming undissociated HF). Adding a source of OH- will neutralize the acid, making the pH increase.
Increase the pH - because more basic
NaF .............. yes, adds F-
SnF2 ........... yes, adds F-
HCl ............... no, adds H+, making it more acidic
NaCl ............. no effect
NaOH ........... yes, neutralizes the acid
H2O .............. no, decreases the concentration of all three species, but shifts equilibrium to right, making more H+
Decrease the pH - becomes more acidic
NaF .............. no, adds F-
SnF2 ........... no, adds F-
HCl ............... yes, adds H+, making it more acidic
NaCl ............. no effect
NaOH ........... no, neutralizes the acid
H2O .............. yes, decreases the concentration of all three species, but shifts equilibrium to right making more H+
