Amount of Substance

Question 1

What’s 1 atm in Pa?

Answer 1

101325

Question 2

How much volume does a gas occupy at STP (standard temperature & pressure)?

Answer 2

22.4 dm^3

Question 3

How can completely different gases occupy the same volume?

Answer 3

Since gas particles are so far apart and barely interact, it doesn’t matter what the gas particles are

Question 4

What’s the equation to finding moles of a gas using volume?

Answer 4

n (mol) =V (dm^3)/24

Question 5

What’s room temperature & pressure in K & Pa?

Answer 5

298 K, 101325 Pa

Question 6

What’s standard temperature & pressure in K & Pa?

Answer 6

273 K, 101325 Pa

Question 7

What’s a standard solution?

Answer 7

A solution of known concentration

Question 8

What’s the name for a ‘solution of known concentration’?

Answer 8

Standard solution

Question 9

Summarise the steps for preparing a known concentration of standard solution of a known volume of sodium carbonate

Answer 9

1. Find mol of sodium carbonate using n=cV and then use Mass = n x Mr to find the mass of sodium carbonate required to make this solution
2. This calculated mass of sodium carbonate is accurately weighed on an electronic balance in a weighing bottle
3. The sodium carbonate is transferred into a volumetric flask with the same volume as the volume asked for in the question; and about 100 ml of deionised water is added, rinsing out the weighing bottle
4. The mixture is shaken in the volumetric flask until the sodium carbonate dissolves
5. Deionised water is added to the volumetric flask up to the mark

Question 10

Summarise the steps for carrying out a titration between sulphuric acid and sodium hydroxide

Answer 10

1. Rinse a burette with H2SO4
2. Fill the burette to the graduation mark ensuring the air is removed from the tap
3. Rinse and fill a pipette with sodium hydroxide, transferring 25 cm^3 to a clean, dry conical flask
4. Add 2-3 drops of indicator
5. Run the acid into the alkali and stop when the colour changes. This is your ‘trial’
6. Record the burette readings to 2 d.p. ending 0/5
7. Repeat the titration until you get 2 concordant results and calculate the mean titre to 2 d.p.

Question 11

What’s percentage yield?

Answer 11

How efficient a reaction is by seeing what percentage of reactants are converted into products

Question 12

Why might a reaction have a percentage yield of less than 100%? (3)

Answer 12

Any from:
- Loss of products from transferring them or from processes such as purification
- Possibility of side reactions
- Impurity of reactants
- Reaction might be at equilibrium

Question 13

What’s atom economy?

Answer 13

How wasteful a reaction is by seeing what proportion of products are useful

Question 14

What’s the name for the measure of how efficient a reaction is by seeing what percentage of reactants are converted into products?

Answer 14

Percentage Yield

Question 15

What’s the name for the measure of how wasteful a reaction is by seeing what proportion of products are useful?

Answer 15

Atom Economy

Question 16

If a reaction has a percentage yield of 100% does that automatically mean atom economy will be 100% too?

Answer 16

No, even if all the reactant are converted into products, not all of the products will necessarily be useful

Question 17

What kind of reactions always have 100% atom economy?

Answer 17

Addition reactions

Question 18

In an exam question, you’re given the volume and concentration of a certain solution. You’re then asked how much water should one add if they were trying to prepare a solution of the same substance but with a target (which is given) lower concentration. How would you answer this question to always get it right?

Answer 18

1. Do Old Conc. divided by New Conc.
2. Multiply this number by the volume given. The number that comes out of this represents the total volume of the new solution
3. Do new volume minus volume given in question (original volume). This is the answer and represents the difference between the total volume of new solution and volume of original solution. Hence, this figure is how much water one should add to achieve the new target concentration

Question 19

In an exam question, a student added a known mass of a known hydrated metal carbonate (Metal Carbonate.xH2O) to a known volume and concentration of a given strong acid in a beaker and stirred the mixture. After the reaction was complete, the resulting solution was transferred to a volumetric
flask, made up to 250 cm^3 with deionised water and mixed thoroughly. Several 25.0 cm^3 portions of the resulting solution were titrated with a known concentration of a given aqueous base. The mean titre of the base was given. Calculate the value of x in Metal Carbonate.xH2O.

Answer 19

1. n(known acid) = cV = given concentration multiplied by given volume
2. n(known base) = cV = given concentration multiplied by given volume
3. Use molar ratio and answer from step 2 to find mol of acid that reacted with this base
4. Multiply this number by 10
5. Now with this number, subtract from it the mol of acid calculated in step 1; the answer to this is the difference between mol of acid at the start of the reaction and mol of acid used in acid-base titration. This difference is the mol of acid reacted with the hydrated metal carbonate
6. Find mole of metal carbonate by stating the equation of the reaction between the given acid and the metal carbonate (dehydrated) and using molar ratios
7. Find the mass of the (would be dehydrated) metal carbonate using Mass = n x Mr
8. The difference between this mass and the mass given of hydrated metal carbonate in the question is the mass of H2O
9. So x = n(H2O) = Mass / Mr