Question

Question 3 (2 points) Barbituric acid, HC4H3N203, is used ito prepare various barbiturate drigs (used as sedaives). Without solving the quadratic equation, calculate the concentration of hydrogen ion in a (2.50x10^-1) M solution of the acid. The value of K, is 9.8x10-5 Express your answer to three significant figures. Note: Your answer is assumed to be reduced to the highest power possible. Your Answer: x10 Answer 54 Question 4 (2 points) What is the concentration of hydroxide ion in a (1.60x10^-1) M aqueous solution of hydroxylamine, HONH2? The Kb for methylamine is 1.1x10-8. You do not need to solve the quadradic equation for this problem. Express your answer to three significant figures. Note: Your answer is assumed to be reduced to the highest power possible. Your Answer: X1 Answer units

Answer 1

3)
HC4H3N2O3 dissociates as:

HC4H3N2O3          ----->     H+   + C4H3N2O3-
0.25                 0         0
0.25-x               x         x


Ka = [H+][C4H3N2O3-]/[HC4H3N2O3]
Ka = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes

Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((9.8*10^-5)*0.25) = 4.95*10^-3

since x is comparable c, our assumption is not correct
we need to solve this using Quadratic equation
Ka = x*x/(c-x)
9.8*10^-5 = x^2/(0.25-x)
2.45*10^-5 - 9.8*10^-5 *x = x^2
x^2 + 9.8*10^-5 *x-2.45*10^-5 = 0
This is quadratic equation (ax^2+bx+c=0)
a = 1
b = 9.8*10^-5
c = -2.45*10^-5

Roots can be found by
x = {-b + sqrt(b^2-4*a*c)}/2a
x = {-b - sqrt(b^2-4*a*c)}/2a

b^2-4*a*c = 9.801*10^-5

roots are :
x = 4.901*10^-3 and x = -4.999*10^-3

since x can't be negative, the possible value of x is
x = 4.901*10^-3

So, [H+] = x = 4.901*10^-3 M
Answer: 4.90*10^-3 M

4)
HONH2 dissociates as:

HONH2 +H2O     ----->     HONH3+   +   OH-
0.16                   0         0
0.16-x                 x         x


Kb = [HONH3+][OH-]/[HONH2]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes

Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((1.1*10^-8)*0.16) = 4.195*10^-5

since c is much greater than x, our assumption is correct
so, x = 4.195*10^-5 M



So, [OH-] = x = 4.195*10^-5 M

Answer: 4.20*10^-5 M