quantitative analysis

Question 1

what is the RFM of water

Answer 1

H - 1 x 2
O - 16 x 1
2+16=18

Question 2

explain how to use reacting masses and then percentage composition to get the empirical formula

Answer 2

With one known mass of one substance react another to it then get the percentages of mass of each element of the overall compound then narrow down to its simplest form for the empirical formula

Question 3

how to you get to the empirical formula

Answer 3

simplest form of each element

Question 4

if you have a substances empirical formula and its relative molecular mass, how would u work out its molecular formula

Answer 4

overall mass divided by the empirical’s RFM

then times that by the empirical formula

Question 5

describe an experiment to devise the empirical formula of magnesium oxide

Answer 5

Place a known amount of magnesium into a crucible which u also know the weight of the heat the crucible under a bunsen for abt 10 mins with the lid on the crucible to stop oxygen escaping allow it to cool then reweigh and see weight of oxygen.
Then divide mass by RAM to get moles
Find lowest ratio between moles (e.g. 1:1 , 1:2)
this is the empirical

Question 6

explain the law of conservation of mass in a closed system

Answer 6

the total mass of the system before and after DOESN’T change

Question 7

law of conservation of mass in a non-enclosed system like a reaction in an open flask that takes in or gives out a gas

Answer 7

If the mass increases: one of the reactants is a gas
- this also works with Mr
If the mass decreases: one of the products is a gas

Question 8

6 grams of magnesium reacts with oxygen to form 10g of Magnesium oxide
How much oxygen was used?

Answer 8

4g fucking obviously

Question 9

What is the concentration equation

Answer 9

grams/volume of solvent(dm3) = concentration

Question 10

what are the 2 things one mole of particles of a substance can be defined as?

Answer 10

a) the Avogadro’s constant number of particles(6.02 x 10 to the 23) of that substance
b) One mole of atoms or molecules of anything will weigh the same as its relative particle mass (Mr, Ar)

Question 11

how many moles are there in 2 grams of calcium?

Answer 11

grams / Ar,Mr = moles
2 grams / 40 = 0.05 moles
To find the number of particles then times the moles by Avogadro’s number

Question 12

I have 2 moles of sodium

how much does it weigh

Answer 12

2 x 23 = 46g

Question 13

why is the mass of the product formed controlled by the mass of the reactant which is not in excess?

Answer 13

The amount of the product formed is directly proportional to the amount of the limiting reactant used

Question 14

Stoichiometry
how many moles of Oxygen will react with 3 moles of magnesium to give Magnesium oxide?

2Mg + O2 –> 2MgO

Answer 14

3 / 2 = 1.5 moles

Question 15

what is the equation for molar concentration

Answer 15

mol/dm-3 = concentration

Question 16

how do u convert from mol dm3 to g dm3

Answer 16

mol/dm3 x Mr = g/dm3

Question 17

how do u calculate percentage yield

Answer 17

actual yield(amount of product)/theoretical yield

Question 18

Give 3 reasons why the actual yield of a reaction is usually less than the theoretical yield

Answer 18

a incomplete reactions
b practical losses during the experiment
c competing, unwanted reactions (side reactions)

Question 19

what is atom economy?

Answer 19

Atom economy is a measure of the amount of starting materials that end up as useful products

Question 20

give the equation for atom economy?

Answer 20

Atom economy = Mr of desired products(from equation / sum of Mr of all reactants (from equation)

Question 21

Why would a particular reaction pathway chosen to produce a specified product?

Answer 21

• minimising the production of unwanted product’s SAVES MONEY
• Some reactions can be more expensive than others e.g. electrolysis
• less production increases sustainability by not wasting resources

Question 22

What is the molar volume of any gas at room temperature and pressure?

Answer 22

Molar volume = 24dm3 or 24000cm3

volume = 24 x moles

Question 23

4K + O2 -> 2K2O

8 grams of Potassium is used how much potassium oxide was created

Answer 23

9.644g

Question 24

what is Avogadros law

Answer 24

V1/n1 = V2/n2
V_{1}	=	first volume
V_{2}	=	second volume
n_{1}	=	first amount of gas (in moles)
n_{2}	=	second amount of gas (in moles)

Question 25

Give a step-by-step for a titration experiment

Answer 25

1) place some alkali into a conical flask using a pipette
2) place the acid into a burette
3) add a suitable indicator (e.g. phenol red)
4) add the acid from the burette to the conical flask until the colour changes
5) note the final reading
6) repeat

Question 26

25.0 cm3 of sulfuric acid reacts with 30.0cm3 of 0.150 mol/dm3 sodium hydroxide. Find the concentration of the acid in both mol/dm3 and g/dm3

Answer 26

moles of NaOH = conc x vol = 0.0045moles (0.150 x 0.03)
moles of H2SO4 = 1/2 x moles of NaOH = 0.00225 mol

conc of H2SO4 = 0.00225. 0.025 = 0.0900 mol/dm3

conc of H2SO4 = 8.82 g/dm3 or 0.0900 mol/dm3