Question

A chemistry graduate student is given 125. mL of a 1.20 M ammonia (NH3) solution. Ammonia is a weak base with Ko=1.8*10. What mass of NH Br should the student dissolve in the NH, solution to turn it into a buffer with pH =9.46? You may assume that the volume of the solution doesn't change when the NH Br is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits.

Answer 1

Given : [Salt 7. 125 ml of 1.20m ammonia (Niz) Solution. Ko of NH3 = 18 8 10 5 7 pK6=-10g (1-8x10-5.) pH = 9:46 To find mass of NH y B? dissolved. for basic buffers, the Henderson Hasselbach equation is pH = 14 - (pkb +log Cate) so; 9.46=14- (0-109 (1.8110-5).+.1og. ). 9:46 = 14-(4-745+ log [Salt] - logi-2) =>9046 = 14-14.745 +10g [Salt] – 0-0792) => 109 [Salt] = 14- 4.745 +0.0792 -9.46: s log Esaht) = -0.1258 -> [Salt] 10 =0.7485. M. Now, molar mass of NH4 Bit is 97.94 g/mol and we get [salt] =0.7485 M . . .= 0.7485 mol/L : now, in 125 mi it is 0.7485 -0-1258 1000 x 125 = 0.094 m so mass of why Br dissolved = 0.0948 97.94 = 9-169 = 9.29

Thus 9.2g NH4Br should be dissolved in NH3 solution.