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# Chemistry buffers help plsssss watch

1. okay so i have these two questions for my chem hwk and i just want to check with someone that ive worked it out correctly
calculate the ph of the following buffer solution:
12.2g of benzoic acid and 7.20g of sodium benzoate in a 250cm3 aqueous solution
Ka for benzoic acid is (6.30x10^-5)

so firstly i worked out the moles of each of them and got:
benzoic acid = 0.1 moles
sodium benzoate = 0.05 moles

from that i then calculated concentrations getting:
benzoic acid= 0.4
sodium benzoate = 0.2

i then plugged this into the equation of [H+] = Ka x [products]/[reactants]
and got an answer of pH 4.5 ((after doin the -log thing)

is this correct???
2. (Original post by ilovethe1975)
okay so i have these two questions for my chem hwk and i just want to check with someone that ive worked it out correctly
calculate the ph of the following buffer solution:
12.2g of benzoic acid and 7.20g of sodium benzoate in a 250cm3 aqueous solution
Ka for benzoic acid is (6.30x10^-5)

so firstly i worked out the moles of each of them and got:
benzoic acid = 0.1 moles
sodium benzoate = 0.05 moles

from that i then calculated concentrations getting:
benzoic acid= 0.4
sodium benzoate = 0.2

i then plugged this into the equation of [H+] = Ka x [products]/[reactants]
and got an answer of pH 4.5 ((after doin the -log thing)

is this correct???
Working out the concentrations is an unnecessary step as the volume is the same for all components.

ka = [H+] x mol salt/mol acid

6.3 x 10-5 x 2 = [H+]

[H+] = 1.26 x 10-4

pH = 3.90

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