Reaction Calculations

Question 1

What is the molecular formula ?

Answer 1

formula that shows the number and type of each atom in a molecule

Question 2

What is the empirical formula ?

Answer 2

simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound
E.g. the empirical formula of ethanoic acid is CH2O

Question 3

What is the gas equation?

Answer 3

PV = nRT
(Pa) (m3) = n (8.31 mol/K) (K)

Question 4

What avogadro’s number?

Answer 4

Number of particles equivalent to relative atomic mass/molecular mass of substance
E.g 6.02 x 10^23 = 12g of Carbon-12

Question 5

Gas volume EQUN?

Answer 5

No. moles = volume of gas (dm3)/ molar gas volume (dm3)
- at room temp/pressure , molar gas volume = 24 dm3
- at standard temp/pressure molar gas volume = 22.4dm3

Question 6

PRACTICAL : making a standard solution

Answer 6

1. Weigh out precise amount of solid
2. Add to a small volume of water/pre dissolve solid
3. Transfer to volumetric flask
4. Make up to SCRATCH mark with water - use pipettes t be precise
5. Rinse the beaker with water/add washings to the volumetric flask
   SWIRL FLASK

Question 7

PRACTICAL : titrations

Answer 7

1. Measure known volume of a solution with volumetric PIPETTE /put into conical flaks
2. Other solution in burette (at 0.00 cm3)
3. Few drops of indicatorto solution
4. Open tap on burette/solution is added to conical flask until changes colour
5. Slow down before end point (add dropwise)
6. Repeat until concordant resultsreached (within 0.2 cm3 of each other)

Question 8

Systematic errors?

Answer 8

errors that occur as a result of a faulty or poorly designed experimental procedure
E.g no reading burette at eye level
- repeating doesn’t remove these errors

Question 9

Percentage uncertainty equn?

Answer 9

(Absolute uncertainty / measured value ) x 100

• times ans by 2 if read instrument 2 times etc

Question 10

% yield equn?

Answer 10

% yield = (actual yield / theoretical yield) /100
- use moles

Question 11

Atom economy equn?

Answer 11

Atom economy = (Mr of desired product / sum of Mr of all reactants ) x 100

Question 12

Molar volume of a gas meaning?

Answer 12

Volume occupied by one mole of a gas at specified temp/pressure

Question 13

Percentage purity equation?

Answer 13

(Actual mass/ expected mass ) x 100

Question 14

Avogadro’s constant equation?

Answer 14

No. Moles = no. Particles / Avogadro’s constant

Question 15

dm3 to m3 ?

Answer 15

Divide by 1000

Question 16

Problem with using water of crystallisation method?

Answer 16

If value of crystallisation is greater than expected - suggest compounds underwent further decomposition
If value is less than expected - not all water molecules driven off during heating

Question 17

PRACTICAL: Determining molar volume of gas

Answer 17

Use HCl and sodium carbonate
1. Measure out fixed vol of HCl into conical flask
2. Add known mass of sodium carbonate to conical flask
3. Immediately connect the gas syringe
4. All reaction to complete/ record vol of CO2 produced
- Repeat with different masses of sodium carbonate
Can use delivery tube/measuring cylinder full of water/measure displacement of water

Use :
Plot graph of vol of CO2 produced against mass of NaCO3
Volume of CO2 produced/moles of sodium carbonate = molar gas volume

Question 18

Assumptions when using gas syringe/delkivery tube method to find molar volume of gas ?

Answer 18

• amount of gas lost between adding sodium carbonate/connecting gas syringe or delivery tube is negligible
• delivery tube/syringe set up is airtight so no gas lost
• reaction is complete