Quantitative chemistry topic 3

Compounds have relative formula mass (M_r) (how to work out)

1. compounds have relative formula mass, M_r, which is just the relative atomic masses of all atoms in molecular formula added together

Calculate % mass of an element in a compound (equation)

Percentage mass of element in compound = A_r x no. atoms of that element/ M_r of compound x 100

Examples of moles (Carbon, Nitrogen gas = N₂, Carbon dioxide = CO₂ )

1. Carbon → A_r = 12, one mole = 12g

2. Nitrogen gas (N₂)→ M_r = 28 (2×14), one mole = 28g

3. Carbon dioxide (CO₂) → M_r = 44 (12+[2×16]), one mole = 44g

In a chemical reaction mass is always conserved

1. No atoms are destroyed or created during reaction

2. same number & types atoms on each side of reaction equation

3. means no mass is lost/ gained → mass is conserved

Mass is conserved example 2Li + F₂ → 2LiF, Show that mass is conserved in this reaction

There’s 2 lithium & fluorine atoms on each side of equation

1. add up relative formula masses on left-hand side of equation:

   2 x M_r (Li) + 2 x M_r (F) = (2×7) + (2×19) = 14 + 38 = 52

2. add up relative formula masses on right-hand side of equation:

   2 x M_r (LiF) = 2 x (7 + 19) = 2 × 26 = 52

3. Total M_r on left-hand side of equation is equal to the total M_r on the right-hand side, so mass is conserved

If the mass increases its probably because one of the reactants is a gas that’s found in air (oxygen) & all products are solids, liquids or aqueous - mass change (before, when reacts, example)

1. before reaction, gas is floating around in air. Its not contained in reaction vessel so cant account for its mass

2. When gas reacts to form part of product it becomes contained inside reaction vessel → total mass of stuff inside the reaction vessel increases

3. Example → metal reacts with oxygen in unsealed container, mass of container increases. Mass of metal oxide produced equals total mass of metal & oxygen that reacted from air

   Metal_(s) + oxygen_(g) → metal oxide_(s)

If mass decreases its probably because one of products is a gas & all reactants are solids, liquid or aqueous - mass changes (before, vessel enclosed?, example)

1. before reaction, all reactants are contained in reaction vessel

2. If vessel isn’t enclosed then gas can escape from reaction vessel as its formed. No longer contained in reaction vessel, so cant account for its mass → total mass of stuff inside reaction vessel decreases

3. Example → metal carbonate thermally decomposes to form metal oxide & carbon dioxide gas, the mass of reaction vessel will decrease if isn’t sealed. In reality mass of metal oxide & carbon dioxide produced will equal mass of metal carbonate that decomposed

   metal carbonate_(s) → metal oxide_(s) + carbon dioxide_(g)

Reactions stop when one reactant is used up ( stops, limiting, directly proportional, more product particles)

1. reaction stops when all of 1 reactant is used up. Any other reactants are in excess - usually added in excess to make sure other reactant is used up

2. reactant that’s used up in reaction → limiting reactant

3. amount product formed is directly proportional to amount of limiting reactant , e.g. double amount limiting reactant = product doubles

4. because add more reactants = more reactant particles to take part in reaction → more product particles

At same temperature & pressure, equal numbers of moles of any gas will occupy same volume. find volume of known mass of any gas

1. at room temperature & pressure (r.t.p = 20C & 1 atom) 1 mole of any gas occcupies 24dm³

2. use formula to find volume of known mass of any gas:

   Volume of gas (dm³)= mass (g) of gas/ M_r of gas x 24

Concentration is a measure of how crowded things are (take place & concentration, more concentrated, concentration g/dm³ & concentration mol/dm³ formula, calculate mass)

1. Lots reactions take place between substances that are dissolved in solution. Amount of substance (mass/ no. moles) in certain volume of solution → concentration

2. More solute (substance thats dissolved) there is in given volume = more concentrated the solution

3. Concentration (g/dm³ ) = mass of solute (g) / volume of solvent (dm³)

4. Can calculate mass by rearranging formula:

   mass = concentration x volume

   Concentration (mol/dm³) = no. of moles of solute (mol) / volume of solvent (dm³)

Might be asked to calculate concentration (titrations, find concentration of other solution)

1. Titrations → experiments that let you find volumes needed for 2 solutions to react together completely

2. Know concentration of 1 of solutions, can use volumes for experiment along with reaction equation to find concentration of other solution

Find concentration of acid in mol/dm³, balanced symbol equation for reaction is : 2NaOH + H₂SO₄ → Na₂SO

1. Work out how many moles of ‘known’ substance you have using formula; no. of moles = conc. x volume

   0.100 mol/dm³ x (25.0/ 1000) dm³ = 0.00250 moles of NaOH

2. using reaction equation to work out how many moles of ‘unknown’ stuff you must have had.

   Can see 2 moles sodium hydroxide : 1 mole of sulfuric acid, 0.00250 moles of NaOH must have reacted with 0.00250 / 2 = 0.00125 moles of H₂SO₄

3. Work out concentration of ‘unknown’ stuff

   concentration = no. of moles / volume

   = 0.00125 mol / (30.0 / 1000) dm³ = 0.041666…mol/dm³ = 0.0417 mol/dm³

Calculate atom economy of following reaction to produce hydrogen gas, CH_4(g) + H₂O_(g) → CO_(g) + 3H_2(g)

1. identify desired product (hydrogen gas)

2. work out M_r of all the reactants:
   M_r(CH₄) = 12 + (4 × 1) = 16

   M_r(H₂O) = (2 × 1) + 16 = 18

   16 +18 = 34

3. work out total M_r of desired product:

   3 x M_r (H₂) = 3 x (2 × 1) = 6

4. use formula to calculate atom economy:

   6/34 × 100 = 17.6%

Negatives of low atom economy ( waste, use up, expensive)

1. making lots of waste = problem

2. use up resources very quickly, at same time make lots waste materials (have to be disposed). makes reactions unsustainable (raw materials run out & waste has to go somewhere)

3. raw materials are often expensive to buy & waste products can be expensive to remove & dispose of responsibility

Ways to fix cons of low atom economy (use waste, more products)

1. find use for waste products rather than throwing them away. Often more than 1 way to make product you want - come up with reactions that give useful by-products

2. the more products there are, the lower atom economy is likely to be