A-Level Chemistry Paper 1 Practicals

How is a volumetric solution made?

• Weigh the sample bottle containing the solid on a (2 dp) balance

• Transfer solid to beaker and reweigh sample bottle.

• Record the difference in mass

• Add distilled water and stir with a glass rod until all the solid has dissolved.

• Transfer to a volumetric flask with washings

• Make up to the 250cm3 mark with distilled water

• Shake flask

What is a standard solution?

A solution of a known concentration

What errors are made when making a volumetric solution?

• If using anhydrous sodium hydrogensulfate (NaHSO4 ), make sure it is not too old as it will have picked up water and therefore mass values will not be as accurate

• Balance – usually gives to 0.01g (systematic error - same for each value that is recorded).

• Alternatively the known mass of solid in the weighing bottle could be transferred to beaker, washed and washings added to the beaker

• Fill so the bottom of the meniscus sits on the line on the neck of the flask. With dark liquids like potassium manganate (K2MnO4 )

• it can be difficult to see the meniscus, so placing a piece of white paper behind to make the marker clearer to see.

How can uncertainties in measuring mass be reduced?

• Using a more accurate balance or a larger mass will reduce the uncertainty in weighing a solid

• Weighing sample before and after addition and then calculating difference (weighing by difference) will ensure a more accurate measurement of the mass added.

How is a titration carried out?

• Pipette used to place 1 reactant into conical flask

• Fill burette with other reactant and reccord initial volume

• Add indicator to the conical flask

• Open the burette tap and allow the reactant to flow into the conical flask and swirl

• Close tap once colour change occurs, using a white tile to see color change

• Record final volume

• Repeat until concordant results and calculate a mean titre

What is weighing by difference

• Using a more accurate balance or a larger mass will reduce the uncertainty in weighing a solid

• Weighing sample before and after addition and then calculating difference (weighing by difference) will ensure a more accurate measurement of the mass added.

How is enthalpy change experimentally calculated?

• Weigh out anhydrous copper sulfate

• Place 25m3 of distilled water into polystyrene cup and record initial temp

• Start timer and record temp each minute for 3 minutes

• At 4th minute add anhydrous copper sulfate, do not record temp

• At 5th minute continue temp readings at minute intervals

• Plot a graph and plot correction graph

What are the errors in experimentally calculating enthalpy change?

• Heat transfer to or from surroundings

• The method assumes all solutions have the heat capacity of water

• Neglecting the specific heat capacity of the calorimeter

• Reaction may be incomplete

• Density of solution is taken to be the same as water

• Water evaporated

• Incomplete combustion

How can accuracy be improved when experimentally calculating enthalpy change?

• An electronic temperature sensor

• Flame calorimeter