Experimental techniques practice questions.

A back titration adds a known excess of one reagent, then titrates the unreacted excess. Why is this method chosen instead of a direct titration?

• The analyte reacts too slowly or is insoluble for a direct titration
• It always gives a more concentrated final solution than a direct titration
• It removes any need to use an indicator at the end point
• It measures the mass of product formed rather than a solution volume

Answer: The analyte reacts too slowly or is insoluble for a direct titration

A back titration is used when the substance cannot be titrated directly, for example an insoluble carbonate or a slow reaction. The amount that reacted is found by subtracting the leftover excess from the amount added.

A student must transfer exactly 25.00 cm³ of a solution for a titration. Which piece of apparatus delivers this fixed volume most precisely?

• A 25 cm³ volumetric pipette
• A 25 cm³ measuring cylinder
• A 50 cm³ measuring cylinder
• A 100 cm³ graduated glass beaker with a mark

Answer: A 25 cm³ volumetric pipette

A volumetric pipette is calibrated to deliver one fixed volume with a small uncertainty, typically ±0.06 cm³. A measuring cylinder or beaker is designed for approximate volumes and has a much larger uncertainty.

Before a titration, a burette is rinsed with a little of the solution it is about to hold rather than with distilled water. Identify the reason.

• Rinsing with the solution removes grease from the inside surface of the glass
• Any distilled water left inside would dilute the solution and make the titre too large
• The rinse warms the burette so that the solution is delivered at a steady temperature
• Any distilled water left inside would react with the indicator and so hide the true end point

Answer: Any distilled water left inside would dilute the solution and make the titre too large

Water clinging to the walls lowers the concentration of the solution actually delivered, so a larger volume is needed to reach the end point. The conical flask is the opposite case: water there does not change the amount of substance already measured into it.

A very fine precipitate clogs a filter paper, so filtration under gravity is extremely slow. Identify the better technique and the reason.

• Decanting the liquid off carefully, which avoids the need to use a filter paper at all
• Filtration under gravity with a larger funnel, which gives the liquid more surface area
• Filtration under reduced pressure with a Büchner funnel, which pulls the liquid through faster
• Warming the mixture thoroughly before filtering, which lowers the viscosity of the liquid throughout

Answer: Filtration under reduced pressure with a Büchner funnel, which pulls the liquid through faster

Reduced pressure below the paper drives the liquid through quickly and leaves a drier solid, which matters when the solid must then be weighed. Warming can help viscosity but would also redissolve many precipitates, which loses the very product being collected.

A reaction mixture must be heated for an hour at the boiling point of the solvent without losing any of it. Identify the correct apparatus arrangement.

• A sealed flask in a water bath, with no condenser attached at all
• Distillation, with the condenser mounted horizontally beside the flask
• An open beaker on a hotplate, topped up with fresh solvent as needed
• Reflux, with the condenser mounted vertically above the flask

Answer: Reflux, with the condenser mounted vertically above the flask

In reflux the vapour rises, condenses in the vertical condenser and runs straight back into the flask, so nothing escapes. The same condenser turned to the side becomes a distillation, which is used when the point is to collect the vapour rather than return it, and a sealed flask would be dangerous.

Concentrated sulfuric acid must be diluted. Identify the correct procedure and the reason for it.

• Add the acid slowly to water with stirring, so the heat released is spread through the bulk
• Add water slowly to the acid with stirring, so the mixture is diluted gradually and safely
• Mix equal volumes of acid and water quickly, so the heat is released in one short burst
• Cool the acid in ice first and then pour the water into it, so that no heat at all is released

Answer: Add the acid slowly to water with stirring, so the heat released is spread through the bulk

Dilution of concentrated sulfuric acid is strongly exothermic, and the large volume of water acts as a heat sink. Adding water to the acid concentrates that heat in a small volume, which can boil the drop and spit concentrated acid out of the container.