Don't get lazy in chemistry. You see what difference there is between a 1m and a 1M solution. By the same token, Hf means heat formation and not HF which I assume you mean hydrofluoric acid. hcl and Naf mean nothing. Start your sentences with a capital letter. To your credit I can tell where the sentences end because you used a period.
100 mL x 0.2M NaF = 20 millimoles.
100 mL x 0.2M HF = 20 mmoles.
4.0mL x 1.0M HCl = 4 mmoles.
.........F^- + H^+ ==> HF
initial..20.....0......20
add............4.0.........
change..-4.0..-4.0....+4.0
equil...16.0...0......4.0
Use the Henderson-Hasselbalch equation, Substitute the equilibrium line from the ICE chart above and solve for pH.
Post your work if you need additional help.
1)100ml sample of solution that is 0.2M in both Naf and Hf has 4.0 ml of 1.0M hcl added to it. calculate the ph change of this soultion .ka =7.2*10^-4
2)40 ml of 0.32M benzoic acid if titrated with 60 ml of 0.2 M naoh. clacilate the ph of the resulting solution at the equivalence point. ka =4.9*10^-9
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