P1- Topic 3-Quantitative chemistry 2022

Question 1

What is the law of conservation of mass? 2

Answer 1

no atoms are lost or made during a chemical reaction

the mass of the products
equals the mass of the reactants

Question 2

Define relative atomic mass and relative formula mass. 2

Answer 2

RAM - average mass of atoms in an element taking into account masses
and abundance of its isotopes, relative to ¹²C

RFM - sum of RAM’s of all atoms in the formula.

Question 3

What is the relative formula mass of CaF₂

Ar values: Ca = 40, F = 19

Answer 3

40 + 19 + 19 = 78

Question 4

Do you use big numbers when calculating relative formula mass

Answer 4

no

Question 5

Reaction occurs in a test tube under a Bunsen
Burner:

4 MgO(s) + CH4(g) → 4 Mg(s) + 2 H2O(g) + CO2(g)

The carbon dioxide and water escape from the test tube.
Use the equation to explain why.

Answer 5

They are both gases

Question 6

how you could increase the precision of the results

Answer 6

Measure to more decimal places (higher resoloution)

use a more sensitive balance /apparatus

Question 7

what is the equation for the percentage by mass of an element ?

Answer 7

total RAM of atoms of that element
——————————— *100
RFM of the compound

Question 8

What is Avogadro’s constant? 2

Answer 8

The number of atoms, molecules or ions in a mole of a given substance.

The value of the constant is 6.02 x 10²³

Question 9

What is the formula that
links mass, molecular mass
and moles together

Answer 9

moles = mass(g)/ RAM or RFM

Question 10

calculate the number of atoms in 1 mole of hydrogen chloride

Answer 10

1 atom of hydrogen and chlorine so two atoms in total

6.0210²³ molecules of hydrogen chloride
so 2 x 6.0210²³

Question 11

State what we mean by a limiting reactant in a chemical reaction 4

Answer 11

In a chemical reaction involving two reactants an excess of one of the reactants is used to ensure that all of the other reactant is used.

completely used up= limiting reactant because it limits the amount of products.

the other is in excess

Question 12

Write down the two formulae that link concentration, mole/mass and volume together.

Answer 12

Concentration (g per dm³) =
Mass(g)/Volume (dm³)

Concentration (mol per dm³)=
nr of moles/volume (dm³)

Question 13

what is meant by concentration

Answer 13

the mass of a solute in a given volume of soloution

Question 14

what gives a higher concentration

Answer 14

a smaller volume or larger mass of solute

Question 15

what gives a lower concentration

Answer 15

a larger volume or smaller mass of solute

Question 16

Why is it not always possible to obtain the theoretical amount of product in a chemical reaction? 3

Answer 16

-reaction not go to completion= reversible

• some of the product may be lost when it is separated from the reaction mixture
• some of the reactants may react in ways different to the expected reaction

Question 17

what is the equation for percentage yield

Answer 17

% Yield =
Actual mass of a product
——————————— x 100%
Maximum theoretical mass of product

Question 18

What is the % yield of NH₃

if 40.5 g NH₃ is produced from 20.0 mol H₂ and excess N₂?

Answer 18

Step 1 - Write the balanced equation
N₂+ 3 H2 → 2 NH₃

Step 2 - Calculate the theoretical amount of NH₃
Moles NH₃ ratio of H₂ to NH₃ is 3:2); of 20/1.5 = 13.3 moles

13.3 X 17 (Mr of NH₃)=227

Step 3 - Calculate percentage yield of NH3
40.5/227 x 100 = 17.8%…

Question 19

What is atom economy?

Answer 19

A measure of the amount of starting materials that end up as useful products

Question 20

what does from the equation mean in atom economy?

Answer 20

the big numbers count so it must be balanced

Question 21

what is the equation for atom economy?

Answer 21

RFM of desired product(from equation)
——————————–*100
Sum RFM masses of all reactants from equation

Question 22

reasons why a particular reaction pathway is chosen/not chosen(atom economy)5

Answer 22

atom economy
 yield
 rate
equilibrium position
usefulness of by-products

Question 23

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

Answer 23

1 mole of a gas at room temperature and pressure occupies

24 dm³

Question 24

what is the equation for volume of gases?

Answer 24

volume= numer of moles*24

dm³

Question 25

how to change from cm³ to dm³

Answer 25

./. by 1000