Question

Calculate the concentration of all species in a 0.145 M solution of H2CO3. The answers are the following: [H2CO3], [HCO−3], [CO2−3], [H3O+], [OH−] = 0.145,2.5×10−4,5.6×10−11,2.5×10−4,4.0×10−11; I am just having the hardest time figuring out how [CO3^2-] was calculated. I keep getting 4.69 x 10^-11M instead of 5.6x10^-11

Answer 1

It all depends upon the Ka2 valoue of the equation

HCO3 +H2O <-------> CO3 + H3O (Ka2 = 4.69*10^-11)

Please mention the Ka2 value of the reactiuon if it is (Ka2 = 4.69*10^-11)

then

again let y = amt of HCO3- that dissociates. [HCO3-] = O - y, where O is from above, 0.0002624 M; then [CO3(2-)] = y. [H+] is primarily determined by the first reaction and is 0.0002624 M

Ka2 = 0.00025*y/[0.00025- y]
[0.00025 - y]*4.69*10^-11 = 0.00025*y
1.17*10^-14 - 4.69*10^-11*y = 0.00025*y
y = 4.69*10^-11 M = [CO3^2-]

Which means you are absolutely correct If the value if Ka2 is any other value then you will get an other value of  [CO3^2-]