To find the molar mass of an unknown weak acid, the number of moles of NAOH deli

Question

To find the molar mass of an unknown weak acid, the number of moles of NAOH delivered to neutralize the acid and the mass of unknown acid must be used. The number of moles NaOH used to neautralize the acid are:

Trial 1:0.002104 mol

Trial 2:0.00185 mol

Trial 3:0.002672 mol

The masses of solid acid used:

Trial1:0.388g

Trial 2:0.363g

Trial 3:0.325g

Use these values to determine the molar mass of the unknown acid. NAOH was 0.1M. original volume of NAOH used was 21.04ml, 18.50ml, and 26.72 ml for each trial respectively.

Explanation / Answer

Let the acid be HA

HA + NaOH => NaA + H2O


(1) Moles of HA = Moles of NaOH = volume x concentration of NaOH

= 21.04/1000 x 0.1 = 0.002104 mol


Molar mass = mass/moles of HA

= 0.388/0.002104 = 184.4 g/mol = 184 g/mol


(2) Moles of HA = Moles of NaOH = volume x concentration of NaOH

= 18.5/1000 x 0.1 = 0.00185 mol


Molar mass = mass/moles of HA

= 0.363/0.00185 = 196.2 g/mol = 196 g/mol


(3) Moles of HA = Moles of NaOH = volume x concentration of NaOH

= 26.72/1000 x 0.1 = 0.002672 mol


Molar mass = mass/moles of HA

= 0.325/0.002672 = 121.6 g/mol = 122 g/mol


Average value of molar mass = (184.4 + 196.2 + 121.6)/3

= 167.4 g/mol = 167 g/mol

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