0.10 moles of sodium cyanate dissolved in 250cm^3 distilled water ,calculate pH?
1 Answer
Explanation:
The idea here is that sodium cyanate,
The cyanate anions will react with water to form isocyanic acid,
Calculate the molarity of the sodium cyanate solution by using the given number of moles and volume. Keep in mind that you need to use liters of solution and that
#"1 cm"^3 = "1 mL" = 10^(-3)"L"#
You will thus have
#color(blue)(c = n/V)#
#c = "0.10 moles"/(250 * 10^(-3)"L") = "0.40 M"#
Sodium cyanate dissociates in a
#"NaOCN"_text((aq]) -> "Na"_text((aq])^(+) + "NCO"_text((aq])^(-)#
This means that the concentration of the cyanate anions will be equal to that of the dissolved salt.
#["NCO"^(-)] = "0.40 M"#
Use an ICE table to help you find the equilibrium concentration of hydroxide anions
#" " "NCO"_text((aq])^(-) + "H"_2"O"_text((aq]) " "rightleftharpoons" " "HNCO"_text((aq]) " "+" " "OH"_text((aq])^(-)#
By definition, the base dissociation constant,
#color(blue)(K_W = K_a * K_b implies K_b = K_W/K_a)" "# , where
Since you didn't provide a value for
https://books.google.ro/books?id=hsuV9JTGaP8C&printsec=frontcover&hl=ro#v=onepage&q&f=false
#K_a = 1.2 * 10^(-4)#
This means that you have
#K_b = 10^(-14)/(1.2 * 10^(-4)) = 8.33 * 10^(-11)#
This will get you
#K_b = (["HNCO"] * ["OH"^(-)])/(["NCO"^(-)])#
#8.33 * 10^(-11) = (x * x)/(0.40 -x ) = x^2/(0.40 -x )#
Because
#0.40 -x ~~ 0.40#
The value of
#8.33 * 10^(-11) = x^2/0.40#
#x = sqrt(8.33/0.40 * 10^(-11)) = 1.44 * 10^(-5)#
Since
#["OH"^(-)] = 1.44 * 10^(-5)"M"#
THe pOH of the solution will be
#"pOH" = - log(["OH"^(-)])#
#"pOH" = - log(1.44 * 10^(-5)) = 4.84#
The pH of the solution will be
#color(blue)("pH" + "pOH" = 14)#
#"pH" = 14 - 4.84 = color(green)(9.16)#
As predicted, the pH of the solution is higher than