Question

A chemistry graduate student is given 500.mL of a 0.60M hydrocyanic acid HCN solution. Hydrocyanic acid is a weak acid with =Ka×4.910−10. What mass of NaCN should the student dissolve in the HCN solution to turn it into a buffer with pH =9.81? You may assume that the volume of the solution doesn't change when the NaCN is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits.

Answer 1

HCN NACN weak acid conjugate base 0.60M ka 9 HON = 4.9 x10-10 bra = -log (ka) = -log (4.9x10-10) = 9.31 volume g solution - soomu þM = 9.81 using Henderson Hasselbach equation pu= pkat log [conjugate base] (weak aced] 9.81 = 9:31+ log CMC CM] (nach] (anting (0.5) = 3k) 0.5 = log [NA CM] [HCN) 3.16 = [Mac] [her] 3:16 [Hen]= [Nacr] 3.16 * 0.00M = [necn] 1.896M = [Mach] Molarity of Na cn = 1.896 M volume y sol - Soome ML = 0.001L 500m2=0.52

mol of NacN = a = molarita Xvolume 1.896 mol x 0.5% = 0.948 mol molar mang NaN = 49.00 g mol man f Nacht = molt molar mass 0.948 mol x 49.00 g = 46. 5g met ang NaON = 479 (in two significant