Hard Water and Heat in a Jodhpur Laboratory
You work in a college laboratory in Jodhpur. In May the room temperature reaches 44°C by noon, and the tap water is hard, with visible scaling in the kettle and a high chloride content. You must prepare 250 mL of 0.1 M silver nitrate solution for a chloride test and 1 L of 1 M sodium hydroxide solution. The chemical store has one 25 g bottle of AgNO3, NaOH pellets, and a carboy labelled “distilled water” that was filled from a local RO plant two weeks ago. The electricity board has announced a six-hour power cut from 10 a.m. to 4 p.m.; the fume hood and air conditioner will not run. A 30% hydrogen peroxide bottle is stored on a sunny windowsill. The teacher wants the solutions by 5 p.m. and asks whether the tap water can be used to save distilled water. Decide and explain.
1. Calculate the mass of AgNO3 and NaOH needed, and decide whether tap water can be used.
Read the worked answer
AgNO3: 0.1 M means 0.1 mol/L. For 0.250 L, moles = 0.0250 mol. Molar mass is about 169.87 g/mol, so mass = 0.0250 × 169.87 ≈ 4.25 g. The 25 g bottle is enough.
NaOH: 1 M for 1 L needs 1 mol × 40.00 g/mol = 40.0 g pellets.
Use distilled water for both. Tap water in Jodhpur is hard and contains chloride; chloride reacts with AgNO3 to form AgCl precipitate, so the silver nitrate solution would be weaker and the chloride test unreliable. Hardness also causes cloudiness in NaOH solution. The opened RO carboy is not ideal for a chloride test; use fresh distilled water or check it first.
2. How should the hydrogen peroxide be stored, and how should the work be scheduled around the power cut?
Read the worked answer
Move the 30% hydrogen peroxide off the sunny windowsill to a cool, dark, ventilated cabinet in its original vented container, away from organic solvents and heat. Heat and light accelerate its decomposition and pressure build-up.
Because the fume hood will be off from 10 a.m. to 4 p.m., do not plan volatile or toxic work in that period. Prepare the solutions before 10 a.m. or after 4 p.m. Dissolve NaOH slowly in cool water because it is exothermic. Store AgNO3 in an amber bottle away from light.
3. What would you tell the teacher about using tap water, and what labelling should go on the two bottles?
Read the worked answer
Tell the teacher that tap water should not be used for the AgNO3 solution, and preferably not for the NaOH solution; use fresh distilled water. Tap water can be used for the first rinse of glassware, followed by a distilled-water rinse.
Label both bottles with the chemical name, concentration, date, preparer, and hazard. NaOH is corrosive; AgNO3 is an oxidizer and stains skin and benches. Keep AgNO3 in the dark.