Question

How many grams of solid ammonium bromide should be added to 1.00 L of a 0.223 M ammonia solution to prepare a buffer with a pH of 9.990 ?

Answer 1

How many grams of solid ammonium bromide should be added to 1.00 L of a 0.223 M ammonia solution to prepare a buffer with a pH of 9.990

Solution :-

Pkb of NH3 = 4.74

Using the Henderson equation lets calculate the concentration of the NH4+ ion needed to get the pH 9.990

pOH= 14 – pH

pOH= 14-9.990 = 4.01

pOH = pkb + log ([NH4+]/[NH3] )

4.01 = 4.74 + log [x/0.223]

4.01-4.74 = log [x/0.223]

-0.73 = log [x/0.223]

Antilog [-0.73] = x / 0.223

0.186 = x/0.223

0.186*0.223 = x

0.0415 =x

So the molarity of the ammonium ion needed = 0.0415 M

So moles of NH4Br needed = 0.0415 mol per L * 1.0 L = 0.0415 mol

Mass of NH4Br = moles * molar mass

                             =0.0415 mol * 97.94 g per mol

                             = 4.06 g NH4Br

So the mass of NH4Br needed = 4.06 g