Chemistry Lab: pH of Buffer Solution Weight about 3.5g NaC2H3O2.3H2O and place i

Question

Chemistry Lab: pH of Buffer Solution

Weight about 3.5g NaC2H3O2.3H2O and place in a beaker.

Add 8.8mL of 3.0 Acetic acid.

Add 55.6mL of distilled water

Stir the solution and measure the PH?

NaC2H3O2.3H2O (FW=136 g/mol)

Pour half of the buffer into another beaker.

lable both beakers 1 and 2

pipet 1.0mL of 6.0M HCl into beaker 1, mix and measure pH?

pipet 1.0mL of 6.0M NaOH into beaker 2, mix and measure pH?

Calculate the pH values of the original buffersolution and the values after the addition of the HCl and NaOH.

How do they compare?

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Paper says :

pH of the orignals buffer 4.5

pH of buffer +HCl 4.2

pH of buffer NaOH 5.8

beaker 1 is 33mL and beaker 2 is 33mL

Show calculations please

Explanation / Answer

pH = pKa + log [salt] / [acid]]

[salt] = mass X 1000 / mol wt X volume (mL)

[salt] = 3.5 X 1000 / 136 X 64.4 = 0.3996 M

[acid]= 0.4099 M

therefore pH of buffer is

pH = 4.76 + log 0.0110 = 4.74

b)

amount of HCl added = 1mL of 6M HCl = 6milllimole = 0.006moles of HCl

This much moles of H+ ions will increase in the solution increasing conc of acetic acid by this amount and same amount of salt will decrease as acetate ions react with H+ ion

therefore new pH will be

pH = pKa + log [salt] / [acid]]

pH = 4.76 + log [0.3996-.006] / [0.4099 + 0.006] = 4.2

on adding NaOH

The amount of NaOH added = 1mL of 6M = 6 millimole of NaOH = .006 moles of NaOH

Thus 0.006 moles of acid will decrease and 0.006 moles of salt will increase

pH = 4.76 + log [0.3996+.006] / [0.4099 - 0.006] = 5

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