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Chemistry/pH- Weak Acid

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Hi again!
I have a new question, Can you help me?
Consider 50.0 mL of a solution of a weak acid HA (Ka = 1.00.E-6), which has a pH of 4.000. What volume of water must be added to make the pH = 5.000?
My Calculations:
To calculate the concentration of x, I take the pH value ->
pH = 4,000=> x=[H+]= 1.E-4=0.0001M=

Ka=(1,0.E-4 )^2/(x-1*E-4)=(1,00.E-6); solving x = 0,0101M

To calculate the quantity of moles in the solution, I do the following:
(50,0*10-3L)* (0,0101M) = 5,05*10-4 mol HA.
I donĀ“t know what to do next:-(

  • Chemistry/pH- Weak Acid - ,

    Go over the approximate solution solution I gave you in the previous question. You can improve the values you get by substituting and solving the quadratic equation,
    Ka = x^2 / c - x)
    The approximate solution is probably adequate. Check the method here:

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