Question

1. How many grams of sodium forrmtate, HCOONa should be added to one liter of 0.150 M formic acid, HCO2H (Ka = 1.8 x 10-4) to prepare a buffer with pH 4.50? bse ntee ibo2o 2 ud to Ho fsu What volumes of 0.045 M benzoic acid (Ka 6.3 X 10-5) and 0.045 M sodium benzoate must be 2. mixed to prepare 1.0 L of a buffer with pH 3.9? 1stlud lo Ha HTo noi ea y uinam 6omulov notuta Shol 916 191ud lo Ha HOGM to notibb ite stu o H 6

Answer 1

1)

Ka = 1.8*10^-4

pKa = - log (Ka)

= - log(1.8*10^-4)

= 3.745

use formula for buffer

pH = pKa + log ([HCO2Na]/[HCO2H])

3.4 = 3.7447 + log ([HCO2Na]/[HCO2H])

log ([HCO2Na]/[HCO2H]) = -0.3447

[HCO2Na]/0.15 = 0.4521

[HCO2Na] = 0.0678

volume , V = 1 L

use:

number of mol,

n = Molarity * Volume

= 6.782*10^-2*1

= 6.782*10^-2 mol

Molar mass of HCO2Na,

MM = 1*MM(H) + 1*MM(C) + 2*MM(O) + 1*MM(Na)

= 1*1.008 + 1*12.01 + 2*16.0 + 1*22.99

= 68.008 g/mol

use:

mass of HCO2Na,

m = number of mol * molar mass

= 6.782*10^-2 mol * 68.01 g/mol

= 4.612 g

Answer: 4.61 g

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