TOPIC 5 - FORMULA, EQ AND AMOUNTS OF SUBST

Question 1

Avogrado’s constant

Answer 1

The number of atoms, molecules or ions in one mole of a substance. 6.02 x 10^23 particles per mole

Question 2

Mole

Answer 2

measures the amount of substance.

DEF - 1 mole of any substance contains the same no. of particles as no. of atoms.

Question 3

Molar mass

Answer 3

The mass of one mole of a chemical with the unit g mol-1.

n = m / M

Question 4

The no. of particles can be calc by…

Answer 4

= no of moles * Avogrado’s constant

Question 5

Which reactions go to completion and what do u do to calc reacting masses?

Answer 5

• Most inorganic reactions.
  so. ..
  1. total combined mass of reactants = total combined mass of products –> LAW OF CONSERVATION
  2. RATIO of species corresponds to the no. of moles

Question 6

Which reactions DO NOT go to completion and what do u do to calc reacting masses?

Answer 6

• Some organic reactions
  so. ..
  1. ASSUME EVERYTHING IS 1:1 RATIO
• USE LIMITING REACTANT AS 1:1 RATIO BY CALC MOLES TO FIND PRODUCTS MOLES AND REMEMBER Cl2 IS Mr Times by 2
• used to calc % YIELD

Question 7

% YIELD

Answer 7

= product formed / theoretical product formed * 100
both in grams

• indicates how efficient a reaction is at converting reactants to products

Question 8

Reasons % yield low

Answer 8

• may have reversible reactions
• some products may be lost when separated from reaction mixture.
• some reactants may react in other reactions

Question 9

Atom economy

Answer 9

of a reaction is % of the total mass of reactants that can in theory be converted into the desired products.

= mass (n*Mr) of desired products/ total mass of products * 100

Question 10

USES -

Answer 10

• how to make a particular useful product

- aware of environmental concerns abt processing and disposing waste

Question 11

IMPROVEMENT OF A.E CAN MEAN

Answer 11

• high A.E, less waste, more efficient product process and preserves raw materials

Question 12

Empirical formula

Answer 12

The simplest whole number ratio of the atoms of different elements in a compound

Question 13

How would you work out the empirical formula?

Answer 13

1. Write down the actual mass or percentage of each element
2. Write down the molar mass of each element
3. Divide the actual mass by the molar mass for each element
4. Divide all the numbers by the smallest one
5. See ratio

to find % of each element in compound then times by 100 of Mr(C/H/O) / Mr of compound

Question 14

What is molecular formula?

Answer 14

The total number of atoms of each element in the molecule

Question 15

How would you work out the molecular formula?

Answer 15

1. Find the molar mass of the empirical formula and divide by relative molecular mass of compound and that no. times by empirical formula

Question 16

How would you work out the molecular formula?

Answer 16

1. Find the molar mass of the empirical formula and divide by relative molecular m ass of compound and that no. times by empirical formula

Question 17

ideal gas equation

Answer 17

pV = nRT

1 atmosphere = 100,000 Pa
1 bar = 100k Pa (100,000Pa)
To convert °C into K... +273 to the number
1m^3 = 1000dm^3 (1,000,000cm^3)
R= 8.314 JK-1mol-1

Question 18

Work out relative formula of MgSO4.nH20 and value of n.

MgSO4.nH2O —> MgSO4 + nH2O

Answer 18

m(H20) = 12.30 -6.0 =6.3g
n(H20) = 6.3/18 = 0.35 mol
N(MgSO4) = 6/120.4 = 0.050 mol 

MgSO4 : H20
0.050/0.050 : 0.35/0.050
1 : 7

Mr [MgSO4. 7H2O] = 246.4g

Question 19

% purity

Answer 19

mass of pure/ mass of impure * 100

Question 20

metal + acid

Answer 20

metal salt + hydrogen
REDOX REACTION
- metals below H in reactivity series do not react
- TEST FOR H2: SQUEAKY POP TEST
light splint and sound heard confirms hydrogen present.

Question 21

metal oxide + acid

metal hydroxide + acid

metal carbonate + acid

ammonia + acid

STRONG ACIDS = MONOPROTIC/DI/TRI…
WEAK ACIDS = partially ionise

Answer 21

metal salt + water

metal salt + water

• insoluble bases form precipitate
  metal salt + water + carbon dioxide

TEST CO2: LIMEWATER TURNS CLOUDY AND PRECIPITATE PROD
- limestone and water react –> limewater and co2
thermal decomp reverse

Ammonium nitrate/chloride/sulfate
test : damp red litmus paper turns blue.

ALL ARE NEUTRALISATION REACTIONS

Acids are PROTON DONORS
Alkaline are PROTON ACCEPTORS

Question 22

Litmus in acid - neutral - alkali
phenolphthalein in acid - neutral - alkali
methyl orange in acid - neutral - alkali

Answer 22

red - purple - blue
colourless - pale pink - bright pink
red - orange - yellow

Question 23

mass concen

Answer 23

= m/v

divide by 1000 if cm3 to dm3

Question 24

work out Mr[ Na2CO3.nH20] and value of n

Answer 24

n(Na2co3) = 5.3/106.0 = 0.05 moles
n(Na2CO3.nH20) = 0.05
Mr (Na2CO3.nH20) =  6.2/0.05 = 124.0
Mr( nH20) = 124.0 - 18.0 
n = 18/18 = 1

ALWAYS 3 SIGFIG ON PAPER

XS(ACID) - UNREACTED ACID = REACTED ACID