Posted by jessica on Tuesday, November 23, 2010 at 2:38pm.
The Ka of hypochlorous acid (HClO) is 3.0 × 108 at 25°C. Calculate the pH of a 0.0385M hypochlorous acid solution?
i got help with this problem, but i don't get where one of the numbers came from (3.4 x 10^5)??? i tried makeing 3.0 x 10^8 the x but i still don't get that number.
start
0.0385
change
x. . . . .. . +x. . . .+x
at equilibrium
0.0385x. .. x..... . .x
Ka = 3.0 x 10^8 = [H+][ClO]/ [HClO]= (x)(x)/ 0.0385x
x = [H+]= 3.4 x 10^5 M
pH =  log 3.4 x 10^5=4.5

chemistry  J, Tuesday, November 23, 2010 at 3:17pm
Hiya,
From what you said it seems that you understand everything up till "x = [H+]= 3.4 x 10^5 M"
So we know that 3X10^8 = x^2/(0.0385x)
Bring (0.0385x) over to the left hand side,
Hence 3X10^8(0.0385x) = x^2
Then we expand the left hand side,
1.155X10^9  3X10^8x = x^2
From there we bring all terms to the right hand side to give
x^2 + 3X10^8x  1.155X10^9 = 0
Then we use our quadratic formula to solve for x > x = [b +/ sqrt(b^24ac)]/2a and reject the negative answer since x = concentration of H+ which cannot be negative. Then we get 3.4X10^5 M.
Alternatively, if the math is a little too tedious, we can approximate x to be near zero since HClO is a weak acid which only dissociates very little in aqueous solution. Then we get 3X10^8 = x^2/0.0385 which should be slightly simpler to solve.
Hope I helped!

chemistry  jessica, Tuesday, November 23, 2010 at 3:26pm
where is 1.155X10^9 from?

chemistry  J, Wednesday, November 24, 2010 at 7:43am
That comes from multiplying 3X10^8 and 0.0385 when we expand 3X10^8(0.0385x).
Answer This Question
Related Questions
 chemistry  The Ka of hypochlorous acid (HClO) is 3.00×108 at 25.0°C. Calculate...
 chemistry  The Ka of hypochlorous acid (HClO) is 3.00×108 at 25.0°C. Calculate...
 Biochemistry  Explain how to prepare 1L of a buffer at pH 7.4 using 1M ...
 Biochemistry  Explain how to prepare 1L of a buffer at pH 7.4 using 1M ...
 chemistry  Ka for hypochlorous acid, HClO, is 3.0*10^(8). Calculate the pH ...
 CHEMISTRY  a) Write the formula for each component in a buffer solution of ...
 Chemistry  A student was required to prepare 250.0 mL of a hypochlorous acid/...
 bio chem  Explain how to prepare 1L of a buffer at pH 7.4 using 1M hypochlorous...
 Chemistry  the ka of hypochlorous acid hclo is 3.5 x 10^8. What is Kb for the ...
 CHEMISTRY!!  1) How would you prepare 20 mL of a solution having pH = 2.00 by ...
More Related Questions