Core Practical 1:Measure the molar volume of a gas

Question 1

Method of Core Practical 1: Measure the molar volume of a gas

Answer 1

1. Measure 30 cm3 of 1 mol dm-3 ethanoic acid and transfer
   to a conical flask.
2. Attach conical flask to gas syringe or use collection over
   water method (see previous page)
3. Measure the mass of a weighing bottle with approximately
   0.05 g of calcium carbonate
4. Add the calcium carbonate to the conical flask- quickly
   resealing the bung so no gas escapes
5. Measure the final total volume of gas
6. Reweigh the empty weighing bottle test tube from step 3
7. Repeat the experiment several more times, increasing the
   mass of calcium carbonate by about 0.05 g each time.

Question 2

What is the objective of Core Practical 1?

Answer 2

To find the volume of one mole of carbon dioxide gas

Question 3

Notes on safety in Core Practical 1

Answer 3

● Wear eye protection.
● Ensure the delivery tube does not become blocked.
● Ethanoic acid will sting if it gets into cuts in the skin.

Question 4

What equipment is needed for core practical 1?

Answer 4

● boiling tube
● stand and clamp
● bung to fit boiling tube, fitted with delivery tube
● water bath for gas collection
● 100 cm3 measuring cylinder
● 50 cm3 measuring cylinder
● test tube
● mass balance (2 d.p.) (Place a test tube rack or 250 ml beaker on the balance to support the test tube while being weighed).
● 1 mol dm−3 ethanoic acid (about 250 cm3 required).
● powdered calcium carbonate

Question 5

What is the overall equation for Core Practical 1?

Answer 5

CaCO3 + 2CH3COOH → Ca(CH3COO)2 + CO2 + H2O

Question 6

Why should you wait until no effervescence is observed before weighing flask containing acid and
marble chips (CaCO3)?

Answer 6

To minimise gas loss

Question 7

Why shouldn’t you use too much calcium carbonate?

Answer 7

so you don’t produce more gas than the measuring cylinder can fit

Question 8

Why is a weak acid used?

Answer 8

so that the reaction is slower, meaning there is less gas loss between adding marble chips and attaching bung.

Question 9

What is an alternative method for core practical 1?

Answer 9

could involve attaching marble chips on a string to the bung, sealing the test tube with a bung, and then tipping the test tube so that the acid comes to the contact with the marble chip. This could reduce the loss of product.

Question 10

What are some possible errors in core practical 1?

Answer 10

● If using syringe, plunger may not be free moving. It may need a lubricant.
● CO2 is slightly soluble in water, so the exact volume is not measured.
● Some gas escapes between addition of marble chips and sealing the test tube.
● Bung may not be airtight.
● Transferring the solid. It is important to weigh the tube containing marble chips
before the addition and reweigh after the addition. This method is ‘ weighing by
difference’ and ensures the amount of CaCO3 that ends up in the reaction mixture is
known..

Question 11

Why should you shake the volumetric flask thoroughly?

Answer 11

to ensure a uniform concentration

Question 12

why should you not heat or put hot solutions in the volumetric flask?

Answer 12

A graduated flask has one mark on the neck which the level to
fill to get the accurate volume. The heat would cause
the flask to expand and the volume would then be incorrect.