practical stuff Flashcards
A student carried out a titration to determine the Mr of a white solid that dissolves in water and reacts with HCl to produce a salt. The student identified the burette as the largest source of uncertainty in this experiment. Using the same apparatus, suggest how the procedure could be improved to reduce the percentage uncertainty in using the burette, justifying why.
• use a larger mass of the solid
• larger titre/reading will be recorded
5 students carried out an experiment to determine the number of water molecules in the formula of hydrated sodium carbonate by heating the sample and recording the change in mass. All the students carried out the experiment exactly according to the method, however one student’s experiment gave an anomalous result - explain why.
• the sample wasn’t heated for enough time
• the sample didn’t lose all its water
The student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling, the student forgets to remove the funnel from the top of the burette.
Suggest why this might affect the titre volume recorded.
Additional drops of solution could have entered the burette from the funnel, making the value on the burette lower.
State one advantage of using a conical flask rather than a beaker for a titration.
Less chance of splashing/losing any solution using a conical flask when swirling
A student determines the enthalpy change for the reaction between calcium carbonate and hydrochloric acid.
The student follows this method:
- measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid using a measuring cylinder and pour the acid into a 100 cm3 glass beaker
- weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into the acid
- stir the mixture with a thermometer
- record the maximum temperature reached.
The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be improved to provide more accurate data.
Describe how this data from the improved method can be used to determine an accurate value for the temperature change.
- use a burette instead of a measuring cylinder
- insulate the beaker
- reweigh the watchglass after adding the solid
- use powdered solid
- measure and record the initial temperature of the solution for a few minutes before adding the solid
- measure and record the temperatures at regular intervals after the solid is added
- plot a graph of temperature against time
- extrapolate to the point of addition
- determine ΔT at the point of addition
Suggest how, without changing the apparatus, a calorimetry experiment involving two solutions could be improved to reduce the percentage uncertainty in the temperature change.
increase concentrations of the solutions (to increase the magnitude of the temperature change)
Why may someone measure to the nearest second in an experiment, as opposed to the nearest 0.01s?
hard to judge to the nearest 0.01s
Sulfur dioxide is produced in an experiment.
Why are small amounts of reactants used in this experiment?
sulfur dioxide is formed, which is toxic
Before adding the solution from the burette in a rough titration, there was an air bubble below the tap.
At the end of this titration the air bubble was not there.
Explain why this air bubble increases the final burette reading of the rough titration.
some solution replaces the air bubble
During a titration, a student washed the inside of the conical flask with some distilled water.
Suggest why this washing does not give an incorrect result.
Doesn’t react with the ___ / doesn’t change the number of moles of ___
A data book value for the enthalpy of combustion of cyclohexane is –3920 kJ mol−1. A student found that through their experiment, the enthalpy of combustion was -1216 kJmol-1.
The student concluded that the temperature rise recorded in the experiment used to investigate the enthalpy of combustion of cyclohexane was smaller than it should have been.
Suggest a practical reason for this.
heat loss
Describe how to prepare 250cm3 of an aqueous standard solution of ____ that contains an accurately measured mass of ____ .
- Weigh out sample in bottle
- Transfer to beaker and re-weigh the bottle (the difference in masses measured in steps 1 and 2 is the mass of ___ in the solution)
- Add distilled water to beaker, and stir with glass rod until all the solid has dissolved
- Transfer beaker contents to a volumetric flask with washings
- Make up to 250cm3 mark on volumetric flask
- Invert volumetric flask (to mix contents together)
A student carried out an experiment to determine the number of water molecules in a sample of hydrated sodium carbonate.
The experimental value was less than the correct value. Suggest why.
Suggest and explain how the procedure, without changing the apparatus, could be improved to give a more accurate value.
• not heated to a constant mass
• heat to a constant mass
• ensures all the water has been driven off
A student does a titration, filling the burette with NaOH solution.
State why the student should use NaOH solution rather than water to rinse the burette.
use of water would dilute the NaOH
The student used a wash bottle containing deionised water when approaching the end-point to rinse the inside of the conical flask.
Explain why this improved the accuracy of the titration.
It ensures all reactants are in the mixture
Suggest one reason, other than incomplete combustion or heat transfer to the atmosphere, why the student’s value for the enthalpy of combustion of methanol calculated from their calorimetry experiment is different from that in a Data Book.
- Experiment not done under standard conditions
- Evaporation of methanol
- Heat capacity of calorimeter not taken into account
The student said correctly that using a thermometer with an overall uncertainty for the rise in temperature of ±0.5 °C was adequate for their calorimetry experiment. The temperature rise in the experiment was 38°C.
Explain why this thermometer was adequate for this experiment.
• The heat loss is a more significant issue
• The temperature rise is much bigger than the uncertainty
Test for ammonium ions
- Add NaOH
- Heat
- Red litmus paper turns blue
Test for magnesium / calcium ions
• add NaOH
• white precipitate formed
Test for strontium / barium ions
• add H2SO4
• white precipitate
The sodium hydrogencarbonate solution neutralises the acid (catalyst), so stops the reaction
burette – fill at/below eye level
dissolving – wear gloves
• remove funnel
• ensure jet is filled (i.e. open tap to fill the space below tap)
• filter
• dissolve in minimum volume of hot solvent
• cool to crystallise, and filter under reduced pressure
• wash with cold solvent, and dry
• reweigh the empty boat
• to calculate the exact mass of salicylic acid added to the reaction mixture
cannot prevent some dissolving
State why reflux is used in this hydrolysis.
reflux allows reactant vapours (of volatile organic compounds) to be returned to the reaction mixture / doesn’t allow any reactant vapour to escape
In a preparation of propanal, propan-1-ol is added dropwise to the oxidising agent and the aldehyde is separated from the reaction mixture by distillation.
Give two ways of maximising the yield of propanal obtained by distillation of the reaction mixture.
liquids are immiscible
liquid goes CLEAR (MS says “ignore colourless”)
the sodium hydrogencarbonate solution neutralises the acid (catalyst), so stops the reaction
• burette – fill at/below eye level
• solution – wear gloves
