Question 6 O out of 4 points Kemmi Major also does a titration. She measures 25.

Question

Question 6 O out of 4 points Kemmi Major also does a titration. She measures 25.00 mL of unknown concentration H2SO4laq) and ttrates it with 0.1000 M NaOHaq). When she has added 42.18 mL of the base, her pheolphthalen indicator turns light pink. What is the concentration (rmolarity of Kemmis H2SO4aal sample? (4 sig hoat dgts, units ln Selected Answer: 0.2109 M Response Use the data for the NaOH to calculate the number of moles of NaOH. Use the acid-base mole ratio to determine the moles of H2S04. Hint - is the mole ratio 1:17) Divide the moles H2504 by the volume in liters to find the molarity.

Explanation / Answer

For a titration between an acid and a base, the volume or concentration of either one is calculated by the volume of the other, at a particular concentration required to completely neutralize the other as VaMa = VbMb.

In this case, 42.18mL of 0.1M NaOH is required to neutralize 25mL of unknown sulphuric acid completely. This complete neutralization is found by the appearance of pink colour by the indicatior phenolphthalein, whose neutral form in acidic medium is colourless and anionic form, in basic medium is pink.

In 42.18mL, no.of moles of NaOH is found by considering that the concentration given - 0.1M shows 0.1mol of NaOH in 1L of solution. So, in 1mL of the solution, there is 0.0001mol and in 42.18mL, there is 0.0001x42.18 = 0.004218mol of NaOH.

Sulphuric acid is diprotic while NaOH is monobasic. So, 2 moles of NaOH is required to neutralize 1 mol of sulphuric acid. So, if 0.004218mol of NaOH was consumed, then 0.004218/2 = 0.002109mol of sulphuric acid is present in 25mL of the sample.

Now, molarity is calculated as no.of moles of a substance in 1L of solution. So, if 0.002109mol of sulphuric acid is present in 25mL of the solution, in 1000mL, there should be 0.002109x40 = 0.08436moles of sulphuric acid should be present [(0.002109/25)x100].

Therefore, the concentration of sulphuric acid in the unknown is 0.08436M.

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