Question

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To make a standard NaOH solution, about 13 mL of 3 M NaOH is mixed with about 300 mL of deionized water (H2OD) in a 500 ml beaker. Why is it not necessary to know the exact volumes of NaOH and H2OD? a b. It isn't necessary to know more than 1 significant digit As a standard solution, the NaOH solution will only need an approximate molarity. The NaOH solution will be further diluted anyway. The NaOH solution will be standardized.

Answer 1

The experiment we are performing is the standadization of NaOH.

Solid NaOH absorbs water from air and it also reacts with CO₂ in air to form Sodium carbonate. As a result we can never accurately determine the concentration of NaOH in a solution as

a. The amount of solid NaOH dissolved in solution has changed because some mass is gained when NaOH absorbs moisture from air

b. Along with NaOH, we also have small amounts of Na₂CO₃ in solution because NaOH reacted with CO₂ to form sodium carbonate.

For these reasons, we standardize a solution of NaOH with potassium hydrogen pthalate (KC₈H₅O₄) or KHP for short. Which means that we find the accurate concentration/molarity of NaOH by titrating a solution of NaOH with KHP.

The prepared NaOH solution (we can only approximate the molarity due to the above mentioned reasons) is filled in a burette. KHP is taken in a flask and the NaOH from the burette is slowly added to the KHP in the flask. We add phenolphthalein as an indicator and when all the KHP has been neutralized by the NaOH, we get a pink color. ( phenolphthalein is colorless in acidic solutions and pink in basic solutions).

By knowing the amount of KHP taken in the flask, we can determine the volume of NaOH solution by noting the burette reading and we can find the accurate molarity or concentration of the NaOH solution.

Q1. Why is it not necessary to know the exact volumes of NaOH and H₂O:

As stated in the above paragraph, the mass of NaOH dissolved in the solution of water cannot be accuraly known. Due to these reasons, the molarity of NaOH will not be known. NaOH solution needs to be further standardized. Answer is d

Q2. Find total volume of NaOH added:

Total volume= FInal burette volume- initial burette volume

= 20.87 mL-0.17 ml = 20.70 mL. Answer is d

Q3. We rinse the sides of the flash with H₂O in order to make sure all the KHP in the flask is neutralized. The water added will not change the amount of H+ ions in solution and therefore will not change the amount of NaOH solution needed to neutralize the KHP acid. Answer is b

Q4. How many moles of KHP reacts with NaOH.

From the given reaction we see that 1 mole of KHP reacts with 1 mole NaOH. Asnwer is b.