Why is strong sodium hydroxide solution stored in a polythene bottle rather than a glass-stoppered one?

It attacks glass and makes the stopper stick

It evaporates quickly through ground glass

It is broken down by light in clear glass

It gives off flammable vapour on standing

Why is sodium hydroxide used only as a secondary standard, standardized by titration before use?

It absorbs water and carbon dioxide from air

It decomposes when dissolved in water

It dissolves too slowly to make solutions

Its molar mass is too low to weigh accurately

A substance suitable as a primary standard must be:

Pure, stable and non-hygroscopic

Standardized against another solution

Hygroscopic, so it dissolves fast

Of the lowest molar mass available

Chemicals: grades, standards and storage

14 Kerala PSC Lab Technician Gr II practice questions on Chemicals: grades, standards and storage, part of Biochemistry. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Why is strong sodium hydroxide solution stored in a polythene bottle rather than a glass-stoppered one?
1. AIt attacks glass and makes the stopper stick
2. BIt evaporates quickly through ground glass
3. CIt is broken down by light in clear glass
4. DIt gives off flammable vapour on standing
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Answer: A · It attacks glass and makes the stopper stick
Strong alkalis slowly dissolve silica, so they etch glass and leach silicate into the solution. Where alkali dries in a ground-glass joint it cements the stopper in place, and the bottle cannot be opened. Polythene bottles, or glass bottles with rubber or plastic stoppers, are used instead. Hydrofluoric acid, which dissolves glass rapidly, must also be kept in plastic.
- B: Evaporation is not the problem; the alkali reacts with the glass.
- C: Sodium hydroxide is not broken down by light.
- D: Sodium hydroxide solution is neither volatile nor flammable.
Question 2 · Easy
Which of these grades of chemical is the purest, suitable for quantitative analytical work?
1. AAnalytical reagent (AR)
2. BChemically pure (CP)
3. CCommercial (technical)
4. DLaboratory reagent (LR)
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Answer: A · Analytical reagent (AR)
Analytical reagent (AR) grade chemicals meet strict purity specifications, and the label states the maximum level of each impurity. They are used for reagents and standards in quantitative work. Laboratory reagent and chemically pure grades contain more impurities and suit general or qualitative work, while commercial (technical) grade is meant for industry. Primary standard grade, purer still, is used when a standard must be weighed out exactly.
- B: CP grade is less pure than AR grade.
- C: Commercial grade is the least pure and is made for industrial use.
- D: LR grade has wider impurity limits than AR and suits routine qualitative work.
Question 3 · Medium
Why is sodium hydroxide used only as a secondary standard, standardized by titration before use?
1. AIt absorbs water and carbon dioxide from air
2. BIt decomposes when dissolved in water
3. CIt dissolves too slowly to make solutions
4. DIts molar mass is too low to weigh accurately
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Answer: A · It absorbs water and carbon dioxide from air
Sodium hydroxide pellets absorb moisture from the air and react with carbon dioxide to form sodium carbonate, so a weighed amount never contains exactly the expected NaOH. Its solution is therefore made up approximately and standardized against a primary standard such as potassium hydrogen phthalate or oxalic acid. Standardized NaOH is kept tightly closed in a polythene bottle and checked regularly. Hydrochloric acid is likewise only a secondary standard.
- B: Sodium hydroxide is stable in solution if protected from carbon dioxide.
- C: Sodium hydroxide dissolves quickly, giving off heat.
- D: A molar mass of 40 g/mol is easily weighed; the problem is that the solid is not pure NaOH.
Question 4 · Hard
To standardize HCl, 0.53 g of anhydrous sodium carbonate (Na2CO3, molar mass 106 g/mol) is dissolved and made up to 100 mL. 10.0 mL of this solution needs 12.5 mL of the acid. What is the normality of the HCl?
1. A0.125 N
2. B0.040 N
3. C0.080 N
4. D0.100 N
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Answer: C · 0.080 N
Sodium carbonate reacts with two H+ ions, so its equivalent weight is 106 ÷ 2 = 53 g. 0.53 g in 100 mL is 5.3 g/L, which is 5.3 ÷ 53 = 0.100 N. From N1V1 = N2V2: 0.100 × 10.0 = N × 12.5, so N = 0.080 N. The acid needed more volume than the standard, so it must be weaker than 0.1 N.
- A: 0.125 N comes from swapping the two volumes in N1V1 = N2V2.
- B: 0.040 N comes from using the molarity of the carbonate (0.05 M) as its normality.
- D: 0.100 N is the strength of the carbonate standard, not of the acid.
Question 5 · Easy
A substance suitable as a primary standard must be:
1. AStandardized against another solution
2. BHygroscopic, so it dissolves fast
3. COf the lowest molar mass available
4. DPure, stable and non-hygroscopic
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Answer: D · Pure, stable and non-hygroscopic
A primary standard is weighed directly to make a solution of exactly known concentration, so it must be of known high purity, stable in air and on drying, and must not take up water or carbon dioxide. A high molar (equivalent) mass is preferred because it reduces the relative weighing error. Anhydrous sodium carbonate and potassium hydrogen phthalate are examples. A solution whose strength is found by titration against a primary standard is a secondary standard.
- A: Standardizing against another solution describes a secondary standard.
- B: A hygroscopic substance gains water while it is weighed, so its mass is wrong.
- C: A high molar mass is preferred, because it reduces weighing error.
Question 6 · Easy
Which grade of chemical is adequate, and cheapest, for preparing a glassware cleaning solution?
1. AGuaranteed reagent (GR) grade
2. BPrimary standard grade
3. CAnalytical reagent (AR) grade
4. DTechnical (commercial) grade
Answer and explanation in LearnFree account
Question 7 · Medium
Picric acid, used in the Jaffe creatinine reagent, is stored moistened with water because when dry it:
1. ACan explode on shock or friction
2. BAbsorbs carbon dioxide
3. CTurns into a carcinogen
4. DLoses its yellow colour
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Question 8 · Hard
An uncapped bottle of anhydrous sodium carbonate, the primary standard for hydrochloric acid, has absorbed moisture. If it is used to standardize HCl, the acid's calculated normality will be:
1. AFalsely high
2. BUnaffected
3. CFalsely low
4. DUnpredictable
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Question 9 · Medium
Why must concentrated nitric acid and potassium permanganate be stored away from alcohol and other organic solvents?
1. AThey are oxidizers that can ignite them
2. BThey absorb solvent vapour and weaken
3. CSolvent vapour makes them light-sensitive
4. DThey react to give toxic chlorine gas
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Question 10 · Easy
Which of these is used as a primary standard to standardize sodium hydroxide solution?
1. APotassium hydrogen phthalate
2. BSodium thiosulphate
3. CConcentrated hydrochloric acid
4. DPotassium permanganate
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Question 11 · Medium
Potassium permanganate solution must be standardized against oxalic acid before use because permanganate:
1. ADecomposes in light and with organic matter
2. BForms an insoluble salt with water
3. CHas no fixed molecular weight
4. DIs too hygroscopic to weigh accurately
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Question 12 · Easy
Metallic sodium is stored in the laboratory under:
1. AWater
2. BDilute acid
3. CKerosene or paraffin oil
4. DEthanol
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Question 13 · Easy
A hygroscopic chemical such as anhydrous calcium chloride is best stored in:
1. AAn open beaker
2. BA water bath
3. CA refrigerator
4. DA desiccator
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Question 14 · Medium
An old, half-empty bottle of diethyl ether is dangerous to distil or heat because on standing ether forms:
1. AToxic cyanides
2. BInsoluble polymers
3. CExplosive peroxides
4. DCorrosive acids
Answer and explanation in LearnFree account

Chemicals: grades, standards and storage

Why is strong sodium hydroxide solution stored in a polythene bottle rather than a glass-stoppered one?

Which of these grades of chemical is the purest, suitable for quantitative analytical work?

Why is sodium hydroxide used only as a secondary standard, standardized by titration before use?

To standardize HCl, 0.53 g of anhydrous sodium carbonate (Na2CO3, molar mass 106 g/mol) is dissolved and made up to 100 mL. 10.0 mL of this solution needs 12.5 mL of the acid. What is the normality of the HCl?

A substance suitable as a primary standard must be:

Which grade of chemical is adequate, and cheapest, for preparing a glassware cleaning solution?

Picric acid, used in the Jaffe creatinine reagent, is stored moistened with water because when dry it:

An uncapped bottle of anhydrous sodium carbonate, the primary standard for hydrochloric acid, has absorbed moisture. If it is used to standardize HCl, the acid's calculated normality will be:

Why must concentrated nitric acid and potassium permanganate be stored away from alcohol and other organic solvents?

Which of these is used as a primary standard to standardize sodium hydroxide solution?

Potassium permanganate solution must be standardized against oxalic acid before use because permanganate:

Metallic sodium is stored in the laboratory under:

A hygroscopic chemical such as anhydrous calcium chloride is best stored in:

An old, half-empty bottle of diethyl ether is dangerous to distil or heat because on standing ether forms: