UNIT 1: SECTION 2 - AMOUNT OF SUBSTANCE

Question 1

number of particles equation?

Answer 1

number of moles x avogadro’s constant.

Question 2

ideal gas equation?

Answer 2

PV = nRT
p = Pa
v = m3
n = moles
r = 8.31
t = K

Question 3

How to make a standard solution?

Answer 3

standard soln. = soln. we know the exact concentration of.

1) use a weighing boat to measure out a sample; reweigh empty boat to find precise mass.
2) empty into beaker - stir till dissolved.
3) empty into volumetric flask and fill with deionised water till 250cm3.
4) stopper and mix abt 10 times.

Question 4

What can we use titrations for?

How?

Answer 4

To work out unknown concentration of acid or alkali.
Pipette unknown soln. into flask.
Few drops of indicator e.g. phenolpthalein.
Fill burette w/ standard soln using funnel below eye level.

Question 5

How do we find molecular from empirical formula?

Answer 5

Find the empirical formula mass.
Mr / Empirical mass.
Multiply empirical formula by this number.

Question 6

How do we find empirical formula from a percentage composition?

Answer 6

we use g/mxr; divide % by Mr.
Divide all the results by the smallest result.
Results are Ratio for empirical formula.

Question 7

How do we find empirical formula from experiments?
E.g. HC burnt in excess of O2,
4.40g CO2 made
1.80g of H2O made.

Answer 7

1. CO2 moles = 4.40/44 = 0.100moles.
2. 1 mole of CO2 contains 1 mole of Carbon so..
   Original HC contains 0.100 moles of C.
3. H2O moles = 1.80/18 = 0.100moles.
4. 1 mole of H2O contains 2 moles of Hydrogen atoms;
   Original HC contains 2 moles of Hydrogen; 0.200
5. Ratio = 0.1:0.2 CH2

Question 8

True or false:

% yield can be 100%

Answer 8

False

Question 9

Equation for %yield?

Answer 9

% yield =

(Actual yield / Theoretical yield ) x 100

Question 10

Define theoretical yield.

Answer 10

Mass of product that should be formed.

Question 11

Why is the actual yield always less than the theoretical yield?

Answer 11

Because some reactants don’t react OR chemicals are lost e.g evaporation.

Question 12

Define atom economy.

Answer 12

The efficiency of a reaction.

Question 13

What does atom economy measure?

Answer 13

How much of the reactant atoms become the desired product.

Question 14

What does % yield measure?

Answer 14

How wasteful a process is.

Question 15

What is the atom economy equation?

Answer 15

(Molecular mass of desired product / sum of molecular mass of products) x 100

Question 16

Why are high atom economies desirable for companies?

Answer 16

Eco-friendly, cost-efficient and sustainable.