Quantitative chemistry

Question 1

What is the conservation of mass?

Answer 1

total mass of the product=total mass of reactant

mass is conserved because no atoms are lost

Question 2

half equation

Answer 2

used to show what happens to one reactant in a chemical reaction
Fe–Fe 2 + 2e-
iron atoms loses two electron

Question 3

What is the relative formulae mass?

Answer 3

relative formulae mass is relative atom mass (top number)
CO2 (12 x 1 ) + (16 x 2) = 44
Ca(NO3)2 40 + (14 x 2) + ( 16 x 6)=164

Question 4

What is Avogadro’s constant and give an example?

Answer 4

6.02 x 10 23
EXAMPLE
Na2O
3 moles so 3 x 6.02 x 10 23=1.806 x10 24

Number of moles=Mass/relative atomic mass
EXAMPLE-calculate the number of atoms in 28g of lithuim Li=7
28/7=4 4 x 6.02 x 1023=2.408 x 10 24

Question 5

What is the equation for Moles?

Answer 5

Number of moles=Mass/relative atomic mass
EXAMPLE-120g of calcium. How many moles of calcium have you ben given. Ar for CA=40
120/40=3moles

Question 6

Limiting reactant example

Answer 6

How many moles of copper will be produced if we used 0.5 moles of copper sulphate and 1 mole of magnesium? Calculate the mass of copper produced

CuSO4 + Mg— MgSO4 + Cu
0.5 x 63.5=31.75g

Question 7

Moles of gas

Answer 7

Volume=amount(mol) x 24dm3

Calculate the volume of 51g of ammonia gas NH3=17
moles=mass/ar
51/17=3
3 x 24=72dm3

Question 8

Concentration

Answer 8

Concentration= mol/volume
A solution of sodium nitrate has a conc of 0.8 mol/dm3.Clculate the mass of sodium nitrate in 0.5dm3.MR NaNO3=85

moles=conc x voulme
moles = 0.8 x 0.5 = 0.4
mass=moles x Ar
0.4 x 85=34g

Question 9

Titeration

Answer 9

17cm3 of hydrochloric acid were required to neutralise 25cm3 of sodium hydroxide.(conc of 0.1 mol) Calculate conc of acid. Mr HCl=36.5

HCl +NaOH–HaCl + H2O

HCL
C= (0.0025/0.017)=0.147 mol/dm3
n=0.0025
V= 0.017 DM3

NaOH
C=0.1 MOL/DM3
N= (0.1 X 0.025=0.0025)
V=0.025 DM3

0.147 x 36.5= 5.366g

n=c x v

Question 10

What are the first steps of titeration?

Answer 10

1)Wash and rinse the pipette with the alkali being used
2)Measure the volume (25cm3) of sodium hydroxide solution using a pipette into a
conical flask

Question 11

What is percentage yield?

Answer 11

Can’t always get 100% yield

• Some product may be lost
• Reversible reaction may not go to completion
• Reaction may react in different ways

Question 12

What is the Percentage yield calculation?

Answer 12

Percentage yield= mass of product actually made/Maximum theoretical product x 100

Question 13

What is the calculation for atom economy?

Answer 13

Atom economy= relative formulae mass of desired product/ sum of all relative formulae mass of all product reactant x 100

Question 14

What factors do you consider when choosing a reaction pathway?

Answer 14

Important factors to consider when choosing a reaction pathway include

• atom economy
• Cost of reactant
• % yield

Question 15

What is the difference between limiting reactant and excess?

Answer 15

Fully used up-Limiting reactant

Not used up-excess reactant

Question 16

What is an ionic equation?

Answer 16

used to simplify complicated equations
show the species(different atoms,molecules or ions involved in a reaction) involved
spectator ions ( ions are not involved with the reaction)are not included

Question 17

Give an example of an ionic equation?

Answer 17

AgNO3 + NaCl—AgCl +NaNO3
silver nitrate +sodium chloride – silver chloride
IONIC EQUATION:
Ag+(aq) + cl-(aq)—AgCl(s)

Question 18

What is an apparent change in mass?

Answer 18

happens when a reaction is carried out in a non-closed system and includes a gas that can enter or leave

Question 19

Give an example of apparent change in mass

Answer 19

when mg is burned in air it forms magnesium oxide so the mass of the solid increases because oxygen has a mass
if the mass of oxygen is included the total mass of all the reactants is equal to the total mass of all the product

Question 20

How do you calculate the mass of a reactant or product from a balance equation?

Answer 20

Al2O3—2Al + 1 1/2 O2
calculate the mass of AL2O3 needed to produce 540g of 2Al

amount of aluminium=540/27=20mol
amount of aluminium oxide required=20/2=10mol
formulae mass of aluminium oxide= (27 x2)+(16 x 3)=102
mass of aluminium oxide needed=10 x 102=1020g

Question 21

What do you add to the sodium hydroxide in the conical flask?

Answer 21

3) Add 5 drops of indicator such as methyl orange to the conical flask
4) Place the flask on a white tile so colour change can be seen clearly

Question 22

What acid do you add to the alkali?

Answer 22

6) Place the sulfuric acid in burette that has been washed and rinsed with acid
7) Add the acid to the alkali until the solution is neutral.Once we see a colour change(yellow-red) add the acid drop by drop until the solution is neutral

Question 23

Why is it important to mix the acid and alkali?

Answer 23

8) Important to swirl the solution to make sure the acid and alkali mix
9) Read the volume of acid added from the burette

Question 24

What do you do once you have done the practical?

Answer 24

10) Repeat the titration x2 take the mean for the final volume
11) Calculate the mean volume of acid added

Question 25

What are common mistakes in this practical?

Answer 25

Make sure you measure the volume in the burette from the bottom of the meniscus

Question 26

What is hydration method to produce ethanol?

Answer 26

during hydration ethene is reacted with steam to form ethanol
atom economy is 100% for this method
addition reaction(all the reactant atom end up in the desired product)

Question 27

What is the fermentation method to produce ethanol?

Answer 27

ethanol can also be produced by the fermentation of glucose

atom economy for this reaction pathway is much lower-only half of the atoms in the reactant end up in desired product

Question 28

What is the theoretical product?

Answer 28

The theoretical yield is the maximum possible mass of a product that can be made in a chemical reaction.