A student from lab obtained 0.723 grams of potassium hydrogenphthalate (KHP, mol

Question

A student from lab obtained 0.723 grams of potassium hydrogenphthalate (KHP, molar mass of 204.2 g/mol). The KHP was added to a 150 mL erlenmeyer flask along with 50 mL of DI water and a few drops of phenolphthalein indicator. The solution was stirred until all KHP was dissolved. The student obtained what was marked as - 0.1M NaOH solution and filled their buret until the meniscus was sitting on 0.50 mL line. The student proceeded to add the sodium hydroxide solution to the KHP solution until the endpoint was reached. The final volume on the buret read 28.83 mL. What was the molarity of the NaOH?

Explanation / Answer

The molarity of KHP , M = ( mass/molar mass) x ( 1000 / Volume in mL)

= ( 0.723 / 204.2) x ( 1000 / 50)

= 0.0708 M

Numer of moles of KHP , n = mass/molar mass

= 0.723 / 204.2

= 3.54x10-3 moles

Volume of the NaOH = final buret reading - initial buret reading = 28.83 - 0.50 = 28.33 mL = 28.33x10-3 L

The balanced equation is : KHC8H4O4 + NaOH ----> NaKC8H4O4 + H2O

So 1 mole of KHP reacts with 1 mole of NaOH

3.54x10-3 moles of KHP reacts with  3.54x10-3 moles of NaOH

So Molarity of NaOH , M = number of moles / volume in L

=  3.54x10-3 moles / 28.33x10-3 L

= 0.125 M

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