2.1.3 Amount of substance Flashcards
Define amount of substance
the quantity that has moles as its units, used as a way of counting atoms
amount of substance can be calculated using mass (n = m/M), gas volumes (n =pV/(RT)) or solution volume and concentration (n = CV).
Define mole (mol)
the amount of substance in grams that has the same number of particles as their are atoms in exactly 12g of carbon-12
Define molar mass
(g mol -1)
the **mass in grams ** in one mole of a substance
MOLAR MASS IS EQUAL TO RELATIVE ATOMIC MASS
The Mr (relative atomic mass) of a substance gives the number of grams in one mole
eg. Mr of Mg is 24.3, so 24.3g of Mg is one mole
(As mole = mass/Mr)
Define molar gas volume
( dm3 mol–1)
the volume occupied by one mole of a gas at a given temperature and pressure
Room temp = 1atm and 25 oC so 24 dm3 mol–1
Define avagadros constant and give the value
The number of particles per mole of a substance
(6.02 x 10 ^23)
So one mole of a substance contains 6.02 x 10^23 particles
144.0g CO2 = 3.27 moles
Molecules = 3.27 x avagadros constant = 1.97 x 10^24
Atoms = 3 (3 atoms) x 1.97 x 10^24 = 5.91 x 10^24
Define empirical formula
the simplest whole number ratio of atoms of each element in a compound
How do you calculate empirical formula ?
- Calculate the moles of each element involved (Moles = mass/Mr)
- Divide all results by the** smallest value** (Round to whole numbers if needed)
- Use this** ratio** to write the new empirical formula
If percentages are given, treat them as if the overall value is 100g
so 56.5 % of 100g will be 56.5 g
Define molecular formula
The actual number of atoms of each element in a compound
How do you calculate molecular formula ?
- Calculate the empirical formula (Moles and then dividing to get a ratio)
- Calculate the Mr of the empirical
- See how many times the Mr of the empirical fits into the actual Mr
- **Multiply ** the empirical by this value
Define hydrated
a crystalline compound that contains water molecules
Define anhydrous
a crystalline compound containing** no** water molecules
Define water of crystallisation
water molecules bonded to the crystalline structure of a hydrated compound
Show the formula of an anhydrous vs a hydrated salt
- CuSO^4 .5H^2O (Hydrated)=
CuSO^4 + 5H^2O (Anhydrous)
In a hydrated substance the amount of water is always shown after a dot
The water present is the water of crystallisation
How do you calculate the** formula of a hydrated salt **from mass data ?
4.76 g Hydrated colbalt (ll) chloride is heated to constant mass 2.60 g
Calculate formula
1. Calculate** moles** of the unhydrated salt and water (Cobalt chloride is 0.02g and water is 1.12g)
2.** Divide moles **by smallest number to get ratio (2.60/0.02 = 1 and 2.16/0.02 = 6
3. These numbers are the formula
4. (COcl^2 . 6 H^2O)
How do you calculate formula of a hydrated salt from percentage composition data ?
Treat the % as masses and calculate formula normally (moles then divide by smallest to get ratio)
Define concentration ?
The amount of solute in moles dissolved per 1dm^3 of solution
Unit = Mol dm^3
What is the equation to calculate concentration ?
Concentration (mol/g dm^3) =
moles or mass / volume (dm^3)
How do you convert from cm^-3 to dm^-3 ?
Divide by 1000
How do you convert from mol dm^-3 to g dm^-3 ?
Mass = mr x moles
So multiply the result by its Mr
eg 0.738 mol dm ^-3 x 74.1 = 54.7 g dm^-3
Define molar gas volume
volume per mole of gas molecules at a stated temperature and pressure
What is the molar gas volume at room temperature and pressure ?
24 dm ^3
What is the equation for volume of a gas ?
This is for ROOM TEMPERATURE AND PRESSURE
Volume (dm^3) =
Moles (mol) x 24 dm^3
What is the ideal gas equation ?
What units are needed ?
This is used when gases are at different temperatures or pressures, or more accuracy is needed
PV = nRT
1. Pressure = Pa (kPa to Pa = x1000)
2. Volume = m^3 ( if from cm^3 divide by 1,000,000, if from dm^3 divide by 1000)
3. Amount = mol
4. Gas constant = 8.314
5. Temp = K (add 273)
How can you use the ideal gas equation to calculate the relative molecular mass of a volatile liquid ?
- Rearrange the equation to get moles
- Convert any units
- Solve for moles
- **Calculate Mr using Mr = mass/moles **
What do the balancing numbers show in a balanced equation ?
the ratio of moles involved in a chemical reaction
(N^2 + 3H^2 = 2NH^3)
- 1 mole of nitrogen reacts with 3 moles of hydrogen to make 2 moles of ammonia)
What can we use balancing numbers to find ?
- quantities of reactants required
- quantities of products that should be formed
How can we use balancing numbers to calculate an unknown quantity ?
- **convert **given quantities into moles (Moles = mass / Mr)
- Use balancing numbers to establish moles of unknown chemical
- Use moles to convert 2nd value into the desired information (mass, volume, concentration)
Define percentage yield
the % ratio of the actual yield of the product compared with the theoretical yield
Theoretical yield = mass of chemical that should be made in a reaction if no chemicals are lost (ACTUAL YIELD IS ALWAYS LESS)
What is the equation for percentage yield ?
Percentage yield =
(Actual yield/theoretical yield) x 100
Give 3 examples why the maximum amount of product may not be achieved
- reaction may not have gone to completion
- other reactions may have occured
- transfer of product may have resulted in some product being lost
Define limiting reactant
completely used up in a reaction (non in excess) which puts a limit on how much product can form
How can you calculate percentage yield while taking into account limiting reactant
Question: Calculate the % yield in this reaction
CH^3C00H + CH^3OH = CH^3COOCH^3 + H^2O
-
Calculate moles of both reactants to find the limiting one
(3.0/60 = 0.05 mol and 2.8/32 = 0.0875 mol)
-First reactant is **limiting **so only 0.05 moles of product are possible -
Calculate maximum theoretical mass using mass = mr x mole
(74 x 0.05 = 3.7g)
3.**Calculate % yield **
(actual/theoretical x100 = 86.5 %)
Define atom economy
a measure of the amount of starting materials that end up as useful products
What is the equation for atom economy ?
( sum of Mr of desired products/ sum of Mr of all products ) x 100
RULE: Balancing numbers must be factored into all equations when calculating the sum of molar masses (Mr)
Give 2 reasons why a high atom economy is beneficial
- Lower costs (less waste of reactants so less money spent on disposal)
- More sustainable (preserves limited resources, less high energy demands-temp, pressure)
