Question

Describe how to prepare 250 mL of 1.00 M ammonia buffer, pH 9.00, starting with 28 wt% NH3 ("concentrated ammonium hydroxide" listed on the back inside cover of the book) and "concentrated" HCl (37.2 wt%) or "concentrated" NaOH (50.5 wt%)

Answer 1

pOH = pkb+log(salt/base)

pOH = 14-9 = 5

pkb of NH3 = 4.74

5 = 4.74+log(salt/base)

(salt/base) = 1.82

salt+base = No of moles = 250/1000*1 = 0.25 mol

No of moles of salt/acid(HCl) = 0.25-0.09 = 0.16 mol

No of moles of base(NH3) = 0.25/2.82 = 0.09 mol

mass of NH3 must be taken = 0.09*17 = 1.53 grams

mass of HCl must be taken = 0.16*36.5 = 5.84 grams

amount of standrad NH3 need to take = 1.53*100/28 = 5.46 grams

amount of standrad concentrated HCl need to take = 5.84*100/37.2

= 15.7 grams

take both sandard amounts of NH3,HCl and dissolve in 250 ml solution by adding enough amount of water.