quantitative chemistry

Question 1

State the law of conservation of mass

Answer 1

• no atoms are lost or made during a chemical reaction,
• so the mass of the products equals the mass of the reactants

Question 2

Define relative formula mass of a compound

Answer 2

• the sum of the relative atomic masses of the atoms in the numbers shown in the formula

Question 3

What are the features of a balanced chemical equation?

Answer 3

• the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown

Question 4

Describe mass change when a reactant or product is a gas

Answer 4

• if​ ​a​ ​reaction​ ​appears​ ​to​ ​involve​ ​a​ ​change​ ​in​ ​mass​ ​it may be ​check​ ​due​ ​to​ ​a reactant​ ​or​ ​a​ ​product​ ​as​ ​a​ ​gas​ ​and​ ​its​ ​mass​ ​has​ ​not​ ​been​ ​taken​ ​into​ ​account​
• reactants may react with air (increase)
• reactants may evaporate (decrease)
• ​(e.g. because​ ​the​ ​gas​ ​has​ ​been​ ​released​ ​into​ ​the​ ​atmosphere)
• e.g. when​ ​a​ ​metal​ ​reacts​ ​with​ ​oxygen:​ ​mass​ ​of​ ​metal​ ​oxide​ ​product >​ ​mass​ ​of​ ​metal

Question 5

When is the mass of a substance numerically equal to its relative formula mass?

Answer 5

• when you have one mole of a substance
  e.g. Mr of iron is 56, so one mole of iron weighs 56g

Question 6

What does one mole of a substance contain in relation to other substances?

Answer 6

• it contains the same number of the states particles, atoms, molecules or ions as one mole of any other substance

Question 7

What are chemical amounts measured in?

Answer 7

• moles (mol)

Question 8

What is Avogadro’s constant?

Answer 8

• the number of atoms, molecules or ions in one mole of a given substance is 6.02 x 10²³

Question 9

How can you convert between moles and grams?

Answer 9

• mass (g) = moles(mol) x RFM (g/mol^-1)
  e.g. moles in 42g of carbon - 42g/12 = 3.5
• moles = mass / Mr

Question 10

What does amounts of substances in balanced equations tell you?

Answer 10

• masses​ ​of​ ​reactants​ ​&​ ​products​ ​can​ ​be​ ​calculated​ ​from​ ​balanced​ ​symbol equations
• chemical​ ​equations​ ​can​ ​be​ ​interpreted​ ​in​ ​terms​ ​of​ ​moles
• e.g.​ ​Mg​ ​+​ ​2HCl​ ​->​ ​MgCl​2​​ ​+​ ​H​2​​ ​shows​ ​that​ ​1​ ​mol​ ​Mg​ ​reacts​ ​with​ ​2​ ​mol​ ​HCl to​ ​produce​ ​1​ ​mol​ ​MgCl​2​​ ​and​ ​1​ ​mol​ ​H​2

Question 11

How would you use moles to balance equations?

Answer 11

• balancing​ ​numbers​ ​in​ ​a​ ​symbol​ ​equation​ ​can​ ​be​ ​calculated​ ​from​ ​the​ ​masses​ ​of reactants​ ​and​ ​products
  1) convert​ ​the​ ​masses​ ​in​ ​grams​ ​to​ ​amounts​ ​in​ ​moles
  2) convert​ ​the​ ​numbers​ ​of​ ​moles​ ​to​ ​simple​ ​whole​ ​number​ ​ratios,​ ​then​ ​you know​ ​how​ ​many​ ​moles​ ​you​ ​have​ ​of​ ​one element/compound​ ​compared to​ ​another

Question 12

What must the total moles of one element be in an equation?

Answer 12

• total​ ​moles​ ​of​ ​one​ ​element​ ​must​ ​be​ ​the​ ​same​ ​on​ ​both​ ​sides​ ​of​ ​the​ ​equation

Question 13

Why should you use a reactant in excess in a chemical reaction involving two reactants?

Answer 13

• to ensure that all of the other reactant is used up

Question 14

Define a limiting reactant

Answer 14

• the first reactant that is completely used up in a reaction (not in excess)
• in a reaction, it is the reactant with the least number of moles

Question 15

What happens if a limiting reactant is used?

Answer 15

• if a limiting reactant is used, the amount of product produced is restricted to the amount of the excess reactant that reacts with limiting one
• (so use amount of limiting reagent not one in excess for calculations)

Question 16

When is the only case where you should include the big number in calculations?

Answer 16

• when calculating atom economy * (you should multiply the big number by the Mr).

Question 17

What happens when the limiting reactant is all used up?

Answer 17

• no more of the product can form and the reaction stops

Question 18

Define the reactant in excess

Answer 18

• the reactant that is left over after the reaction stops;
• this happens because the limiting reactant has been used up

Question 19

Define concentration in terms of volume

Answer 19

• the amount of solute dissolved in a given volume (dm³) of solution

Question 20

What can the concentration of a solution be measured in?

Answer 20

• mass per given volume of solution (g/dm³)
• mol. per given volume of solution (mol.dm³)

Question 21

How would you calculate mass / moles of solute in a given volume of a known concentration?

Answer 21

• mass = conc x volume
• moles = conc x volume

Question 22

How would you calculate the concentration of a solution?

Answer 22

• conc = mass of solute / volume solvent

Question 23

How would you calculate the conc of the other solution if the volumes of two solutions that react completely are known where the conc of one solution is known?

Answer 23

• work​ ​out​ ​moles​ ​of​ ​solution​ ​where​ ​volume​ ​and​ ​concentration​ ​is​ ​known by​ ​moles​ ​=​ ​conc​ ​x​ ​vol​ ​(make​ ​sure​ ​volume​ ​is​ ​in​ ​dm​3,​ ​ ​to​ ​go​ ​from​ ​cm3​ ​​ ​to dm​3​​ ​÷​ ​1000)
• work​ ​out​ ​moles​ ​of​ ​other​ ​solution​ ​by​ ​mole​ ​ratio​ ​from​ ​equation
• now​ ​work​ ​out​ ​unknown​ ​concentration​ ​by​ ​using​ ​conc​ ​=​ ​mol​ ​/​ ​vol

Question 24

What gives a higher / lower concentration?

Answer 24

• a smaller volume or larger mass of solute gives a higher conc
• a larger volume or smaller mass of solute gives a lower conc

Question 25

How would you calculate percentage yield?

Answer 25

• percentage yield = amount of product produced / maximum of product possible (x 100)

Question 26

Why is it impossible to get 100% yield?

Answer 26

• the reaction may not go to completion because it is reversible
• some of the product may be lost when it is separated from the reaction mixture - e.g. some left in the apparatus
• some of the reactants may react in ways different to the expected reaction
• the reaction may not have fully completed

Question 27

Define yield

Answer 27

• amount of product obtained

Question 28

How would you calculate the theoretical mass of a product from a given mass of reactant ant the balancing equation for the reaction?

Answer 28

• calculate​ ​mol.​ ​of​ ​reactant​ ​by​ ​using​ ​mol.​ ​=​ ​mass​ ​/​ ​molar​ ​mass
• use​ ​balancing​ ​numbers​ ​to​ ​find​ ​mol.​ ​of​ ​product​ ​(e.g.​ ​2HCl​ ​+​ ​Mg​ ​->​ ​MgCl​2 if​​ you ​​have​ ​2​​mol. ​​of ​​HCl,​​you​​ would​​ divide​​ by​​ 2​​ to​​ get​ ​1​​mol.​​of​ ​Mg Cl​2.​)
• calculate​ ​theoretical​ ​mass​ ​of​ ​a​ ​product​ ​by​ ​then​ ​using​ ​mass​ ​=​ ​mol.​ ​x molar​ ​mass

Question 29

Define atom economy

Answer 29

it’s a measure of the amount of starting materials that end up as useful products

Question 30

What is high atom economy important for?

Answer 30

• it is important for sustainable development and for economic reasons to use reactions with high atom economy

Question 31

How would you calculate atom economy?

Answer 31

• atom economy = Mr of desired product from reaction / sum of Mr of all reactants (x 100)

Question 32

What is the rule for equal amounts in moles of gases?

Answer 32

• equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure

Question 33

Define molar gas volume

Answer 33

• the volume occupied by one mole of any gas at room temperature and pressure (20C and 1 atmosphere pressure) is 24dm³

Question 34

How would you calculate percentage mass of an element in a compound?

Answer 34

• Ar x number of atoms of that element divided by Mr of compound then x 100

Question 35

How would you calculate the volume of gas (dm^3)?

Answer 35

• volume of gas at RTP = mass of gas / Mr of gas x 24
• e.g.​ ​if​ ​you​ ​produce​ ​5​ ​moles​ ​of hydrogen,​ ​you​ ​produce​ ​24​ ​x​ ​5​ ​=​ ​120​ ​dm3​ )​