PRACTICAL SKILLS

Question 1

Describe the practical to find the molar volume of a gas (core1)

Answer 1

1) Measure 30cm3 of 1 mold dm -3 ethanoic acid and transfer to a conical flask
2) Attach a bung to the conical flask and insert delivery tube of bung underneath an inverted measuring cylinder in a trough of water
3) Place 0.05g of calcium carbonate in a test tube and measure contents of calcium carbonate plus test tube
4) Add the calcium carbonate into the flask- quickly resealing the bung so no gas escapes
5) After reaction is complete, measure the final total volume of gas produced
6) Reweigh empty test tube that originally had the calcium carbonate in
7) Repeat experiment several more times, increasing mass of calcium carbonate by 0.05g each time

Question 2

Describe the graph you would get when you plot calcium carbonate mass ( x axis) against volume of gas produced (y axis)

Answer 2

A directly proportional graph - a straight line through the origin

Question 3

Give a reason why the student added a small pinch of calcium carbonate to the acid before starting the reaction.

Answer 3

To saturate the the solution with CO2 / to stop CO2 gas formed from dissolving

Question 4

How would you work out molar gas volume of molar gas volume of CO2 gas

Answer 4

From the graph read the volume of CO2 given off with eg: 0.25 g CaCO3
Work out the moles of CaCO3
in 0.25g = 0.25/100.1 = 2.5 x 10-3
Assume the moles of CO2 = moles of CaCO3
Work out molar volume of CO2 = volume of CO2
/ moles of CO2

Question 5

Describe how to make a standard solution (core 2)

Answer 5

1. Weigh an empty test tube. Scoop approximately 2.5 g of sulfamic acid into the test tube.
2. Reweigh the test tube and its contents accurately.
3. Dissolve the sulfamic acid in approximately 100 cm3 of distilled water in a beaker.
4. Transfer the solution, including the washings, into a 250 cm3 volumetric flask and make the solution up to the mark with deionised water. Use a pippette for the last few drops. Invert to mix
   –> bottom of the meniscus should sit on the line

Question 6

Why is using a pipette more accurate then a measuring cylinder

Answer 6

A pipette has a smaller uncertainty

Question 7

Describe how to measure the mass of a solid

Answer 7

1. Measure mass on 2 or 3d.p. balance of a weighing bottle
   with the required quantity of solid in it
2. Empty mass into reaction vessel/flask
3. Reweigh the now empty weighing bottle
4. Subtract the mass of the empty weighing bottle from the
   first reading to give exact of mass actually added.

Question 8

Describe the acid based titration using hydrochloric acid and sodium hydroxide practical (core 3)

Answer 8

Wash out the volumetric flask with distilled water then hydrochloric acid
Use a pipette and pipette filler to measure 25 cm3 of HCl (hydrochloric acid) and transfer this to volumetric flask
Make up the rest of the solution to 250cm3 with deionised water. Invert to mix.
Using pipette and pipette filler, pipette a 25cm 3 aliquot (portion) of the now dilute hydrochloric acid into a conical flask
Add 2-3 drops of phenolphthalein indicator
Wash out burette with distilled water then sodium hydroxide, then fill it up, ensuring the burette is filled all the way to the tap with no air bubbles
Preform the titration: all burette readings should be to the nearest 0.05
Swirl the flask and add alkali drop by drop once titration reaches its endpoint
Repeat titration until concordat results