physical practical skills questions Flashcards
another student is required to make up 250cm3 of an aqueous solution that contains a known mass of MHCO3. The student is provided with a sample bottle containing the MHCO3.
Describe the method, including apparatus and practical details, that the student should use to prepare the solution.
step 1: transfers known mass of solid
a) weigh the sample bottle containing the solid on a 2dp balance
b) transfer to beaker and reweigh the sample bottle
c) record the difference in mass
step2: dissolves in water
a) add deionised water
b) stir with glass rod until all solid has dissolved
step 3:
a) transfer to VOLUMETRIC FLASK
b) with washings
c) make up to 250cm3 mark with water
d) Shake
the student identified use of the burette as the largest source of uncertainty in the experiment.
using the same apparatus, suggest how the procedure could be improved to reduce the percentage uncertainty in using the burette.
Justify your suggested improvement
SUGGESTION:
use more concentrated solution of MHCO3 (the white solid which is to be dissolved in HCl)
JUSTIFICATION:
so that larger titre reading will be needed
the students followed the method:
measure out 50cm3 of 1.00moldm-3 aqueous HCl using a measuring cyclinder and pour the acid into 100cm3 glass beaker
weigh out 2.50g of solid calcium carbonate on the watch glass and tip the solid into the acid
stir the mixture with a thermometer
record the maximum temperature reached.
the student uses data to determine a value doe the enthalpy change
explain how the experimental method can be used to determine an accurate value for the temperature change
stage 1: APPARTUS
a) use the burette/pipette (instead of measuring cylinder)
b) use a polystyrene cup (instead of beaker)
c) reweigh the watch glass after adding the solid
stage 2: TEMPERATURE MEASUREMENTS
a) measure and record the initial temperature of the solution for few minutes before addition
b) measure and record the temperatures
stage 3: TEMPERATURE DETERMINATION
a) plot a graph of temperature against time
b) extrapolate to the point of addition
c) determine temperature change at the point of addition
d) use temperature change to work out Q (energy released)
d) Q divide n = enthalpy change
importance of percentage yield
- work out how to get as much product as possible from the reaction
- efficient conversion of reactants to products
importance of atom economy
idea of maximising the mass of reactants/ atoms that ends up in desired product
minimising the amount of by-products
state in terms of fundamental particles, one similarity and one difference between atoms of 50Cr and 53Cr
SIMILARITY = both atoms have the same number of protons/electrons
DIFFERENCE = atoms have a different number of neutrons
give two reasons why it is necessary to ionise the isotopes of Chromium before they can be analysed in the time of flight mass spectrometer
- ions will interact with and be accelerated by an electric field
- ions create a current when hitting the detector
electrospray ionisation
- sample is dissolved in volatile solvent (solvent must contain H+ , H20, Methanol)
- injected through a fine hypodermic needle giving a fine mist
- tip of needle is high voltage
- sample each gains a proton as it leaves the needle
electron impact ionisation
high energy electrons
fired through an electron gun
fired at sample
knocking off one electron
to form a positively charged gaseous ion
give the equation for the electrospray
ionisation of P
P + H+ → PH+
state why calibrating a pH meter just before it is used improves the accuracy of the pH measurement
OVERTIME/ IN STORAGE meter doesn’t give accurate readings
describe how you would obtain the pH curve for the titration
- measure pH (of the acid)
- add alkali in know small portions (1cm-2cm3)
- stir mixture
measure pH (after each addition) - repeat until alkali in excess (27-50cm3)
- add in smaller increments near endpoint ( 0.1-0.5cm3)
the student uses a funnel to fill to 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 funnel
state one advantage of using a conical flask rather than a beaker for the titration
less chance of losing/ spilling any solution
the student rinsed the burette before filling it with the sodium hydroxide solution rather than water for the final rinse of the burette.
using water would DILLUTE the NaOH / change the concentration of NaOH.
