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3/26/2017 05:00 PM A 5.4/10 3/26/2017 02:17 AM Gradebook Print Calculator Periodic Table Question 5 of 9 Map A Sapling Learning A student prepares a standardized solution of sodium hydroxide that is about 0.1 M and stores it in a plastic bottle. To determine the exact concentration she puts the NaOH solution in a buret and titrates a carefully-weighed solid acid of known molar mass that has been dissolved in deionized water. She records the volume of NaOH needed to neutralize the acid and repeats the process several times to determine the concentration of the NaOH solution. She calculates the concentration from the titration data. Why is a plastic bottle generally preferred to a glass bottle to store NaoH? o Sodium hydroxide reacts slowly with glass, forming Sodium hydroxide renders plastic inert by a process called passivation. The reaction between sodium hydroxide and plastic produces plastic hydroxide. O Sodium hydroxide reacts with polyethylene. Why is potassium acid phthalate (KHP) (KHC8H404, MM 204.227 g/mol) sowidely used as a primary standard in acid-base titrations? Select all that apply t is available in high purity It easily absorbs moisture from the atmosphere It has a relatively high molar mass. It is an acid-base indicator. t is stable to air. Scroll down. t is expensive There are 5 parts to this question. A Previous Give Up & View Solution Check Answer Next Ext Hint

Explanation / Answer

1) Glass contains silica, SiO2. Silica is acidic in nature and NaOH, being a base, reacts with silica to form a salt, sodium silicate as per the equation below:

2 NaOH + SiO2 ------> Na2SiO3 + H2O

Hence, the correct answer is the first option. Sodium hydroxide reacts slowly with glass, forming sodium silicate.

2) Potassium acid phthalate is used as primary standard because it can be accurately weighed and the weight of potassium acid phthalate doesn’t change due to reactions with moisture (infact potassium acid phthalate doesn’t react with moisture or air). It is available in high purity and is inexpensive. Moreover, it has high molar mass; hence little errors introduced in the weighing of potassium acid phthalate will not contribute significantly to a gross error in the concentration calculation of NaOH. Hence the reasons are:

a) It is available in high purity.

b) It has a relatively high molar mass.

c) It is stable to air.

d) It is expensive.

3) Write down the reaction between potassium acid phthalate and NaOH as below:

KHP + NaOH ------> NaKP + H2O

At the equivalence, we have

Moles of NaOH added = moles of KHP added.

Hence, at the equivalence point, the amount of NaOH depends on the millimoles of KHP and not on the volume of the solution. Therefore, the last option is the correct answer.

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