The weak acid HQ has pKa of 4.89.
1)Calculate the [H3O+] of .035 M HQ.
2)Calculate the (OH-) of 0.500 M HQ
....Am I correct?
a) pKa = 4.89
=> - log(Ka) = 4.89
=> Ka = antilog(- 4.89) = 10-4.89 = 1.29x10-5
1. Given concentration of HQ, C = 0.035 M
Dissociation of HQ is given as
HQ(aq) -- > H3O+(aq) + Q-(aq), Ka = 1.29x10-5
Initial conc:0.035 M 0 0
eqm conc: (0.035-x)M x M x M
Ka =([H3O+(aq)]x[Q-(aq)]) / [HQ(aq)] = (x M * x M) / (0.035-x)M
Since x < < 1, (0.035-x) is nearly equals to 0.035
=> 1.29x10-5 = x2 / 0.035
=> x = Underroot (0.035 x 1.29x10-5 ) = 6.72x10-4
x = [H3O+(aq)] = 6.72x10-4 (answer)
B)for 0.500M HQ:
k_a=(x.x )/(0.5-x)=¡¼10¡½^(-4.89)=1.26*¡¼10¡½^(-5)
x=0.00251
pH= -log0.00251=2.6
But
pH+pOH=14
pOH=11.4
[OH^- ]=¡¼10¡½^11.4=2.5*¡¼10¡½^11
Thank you.
a is ok.
b I obtained 2.54 for pH with pOH 11.46
To calculate the [H3O+] of 0.035 M HQ, you correctly found the Ka value, which is 1.29x10-5. Next, you set up an equation using the Ka expression: Ka = (x^2)/(0.035 - x), where x represents [H3O+]. Since x is much smaller than 0.035, you can approximate 0.035 - x as 0.035.
You then solved for x by plugging in the values: 1.29x10-5 = x^2 / 0.035. By rearranging the equation and taking the square root, you found x to be 6.72x10-4, which represents the concentration of [H3O+]. Great job, your answer is correct.
To calculate the [OH-] of 0.500 M HQ, you first found the Ka value as 1.26x10-5. Then you set up an equation using the Ka expression: Ka = (x^2)/(0.5 - x), where x represents [H3O+].
However, it seems there was a mistake in your calculation. If you recalculate using the correct equation and Ka value, you should obtain x = 0.00251 for the concentration of [H3O+].
To find the [OH-] concentration, you can use the equation pH + pOH = 14. Since you found the concentration of [H3O+], which gives you pH = -log([H3O+]), you can subtract the pH value from 14 to get pOH. Then, using the equation pOH = -log([OH-]), you can calculate the concentration of [OH-].
By correcting your calculations and following these steps, you should find the correct value for [OH-]. Good luck!