Question

Part A Calculate the concentration of an aqueous solution of Ca(OH), that has a pH of 11.25. Express your answer using two significant figures. PO AE O a ? [Ca(OH)2) = M Submit Request Answer Provide Feedback Doorn

Answer 1

Let the initial concentration of Ca(OH)2 be Co M.

Ca(OH)2 will dissociate as:-

Reaction	Ca(OH)2(aq)	$\rightarrow$	Ca2+(aq)	+	2OH-(aq)
Initial Concentration	Co M		-		-
Change in Concentration	-x M		+ x M		+ 2x M
Final Concentration	(Co - x) M		x M		2x M

Since, Ca(OH)2 is a strong base, therefore, it will dissociate completely and no Ca(OH)2 will be left behind in undissociated form.

Therefore, the final concentration of Ca(OH)2 = (Co - x) M = 0 M

So, x = Co M

Hence, [OH-] = 2x M = 2Co M

It is given that the pH of this solution is 11.25

So, pH = 11.25 and pH + pOH = 14

Therefore, pOH = 14 - 11.25 = 2.75

So, [OH-] = 10-pOH M = 10-2.75 M = 100.25 x 10-3 M = 1.8 x 10-3 M (100.25 = 1.78)

Now, 2Co = 1.78 x 10-3 M

or Co = (1.78 x 10-3/2) M

Co = 8.89 x 10-4 M

For 2 significant figures, Co = 8.9 x 10-4 M (rounded off upto 1 decimal place)

Therefore, the concentration of Ca(OH)2 to achieve the given pH is 8.9 x 10-4 M.