Unit 1 Stoichiometry

Question 1

What are the 4 states of matter

Answer 1

Liquid solid gas and plasma which is an ionised gas. It is mainly found in outer space.

Question 2

what is fractional distillation

Answer 2

Fractional distillation is a technique that is used to separate the components of a mixture based on their different boiling points.

Question 3

define homogenous mixtures and heterogenous mixtures

Answer 3

A homogeneous mixture has the same composition throughout.

a heterogeneous mixture has a non-uniform composition.

Question 4

outline the relationship between mole and avogadro constant

Answer 4

The number of particles (or elementary entities) in a mole of a substance is numerically equal to the Avogadro constant

A mole of any substance contains the same number of particles, although moles of different substances have different masses.

Question 5

how do you find the number of atoms in one mole of something molecules

Answer 5

multiply the number of atoms in one molecule by the Avogadro constant

Question 6

how do you convert from number of particles to amount in mol and vice versa

Answer 6

number of particles divide by avogadros constant = amount in mol

amount in mol multiplied by avogadros constant = number of particles

Question 7

How do you find mass of one molecule

Answer 7

Molar mass of substance divided by avogadro’s constant

Question 8

How to find empirical formula from percentage composition by mass

Answer 8

1. Assume percentage equals mass of each element in 100g compound.
2. Divide mass in grams by molar mass to find moles
3. Divide all results by lowest mol
4. Multiply all results if one answer is a decimal

Question 9

How do you find molecular formula based on molar mass and the empirical formula

Answer 9

1. Calculate the mass of the empirical formula using relative atomic masses
2. Divide molar mass by the mass of the empirical formula
3. Multiply empirical formula with results to get molecular formula.

Question 10

How do you calculate the percentage composition

Answer 10

Mass of element in compound divided by molar mass of compound times 100.

Question 11

Explain the method of calculating the empirical formula of a compound from an experiment including a crucible
And include possible error in the method

Answer 11

1. Determine mass of compound used in the reaction.
2. Determine mass of other compound or whatever it reacted with
3. Convert from mass in gr to amount in mol for both masses.
4. Divide both by the smallest amount in mol
5. Determine ratio by rounding results if necessary

Errors possible:
1. Compound wasn’t pure
2. Product was something other than what used to calculate with
3. Some product was lost when the lid was removed to allow oxygen in.

Question 12

Explain how to determine empirical formula of a compound based on a combustion analysis

Ex. Question: A 0.250 g sample of a compound containing carbon, hydrogen and oxygen undergoes complete combustion to produce 0.366 g of CO2 and 0.150 g of H2O. Determine the empirical formula of the compound.

Answer 12

1. Determine mass in gr of C in 0.366g of CO2 and mass in gr of hydrogen in 0.150g H2O.
2. Determine the mass in g of oxygen in the sample by subtracting the mass of the C and H from the total mass of the sample.
3. Convert from mass in gr to amount in mol for the C, H and O
4. Decide each by smallest amount in mol
5. Determine empirical formula using ratios.

Question 13

Explain the method of determining the limiting and excess reactants

Ex: Iron(III) oxide reacts with carbon to produce iron and carbon dioxide according to the following equation.

2Fe2O3 (s) + 3C (s) → 4Fe (s) + 3CO2 (g)

500.0 g of iron(III) oxide is reacted with 500.0 g of carbon. Determine the limiting and excess reactants

Answer 13

1. Convert iron and C from mass to mol, using n=m/mr

To find limiting reactant:
2. Divide each amount in mol by its coefficient in the balanced equation
3. The lowest value is the limiting reactant and the highest value is the excess reactant

Question 14

What is the theoretical yield and why is it hardly achieved

Answer 14

The theoretical yield is the maximum amount of product that can be produced assuming that all the limiting reactant has reacted.

solutions sticking to the glassware, precipitates that remain on the filter paper or the loss of small amounts of product.

Question 15

What is the actual yield

Answer 15

The amount of product that is actually produced in a chemical reaction is the actual yield.

Question 16

How do you calculate the percentage yield

Answer 16

Actual yield / theoretical yield ) times 100

Question 17

How do you calculate the theoretical yield

Ex: Acetylsalicylic acid, also known as aspirin, C9H8O4, is synthesised by reacting salicylic acid (C7H6O3) with ethanoic anhydride (C4H6O3) according to the following equation:

C7H6O3 (s) + C4H6O3 (l) → C9H8O4 (s) + C2H4O2 (l)

1. Calculate the theoretical yield, in g, of aspirin, when 3.00 g of salicylic acid are reacted with 4.00 g of ethanoic anhydride.

Answer 17

1. Convert from mass in grams to amount in mol for both
   C7H6O3 and C4H6O3
2. Determine the limiting reactant in reaction by dividing each amount in mol by the coefficient in balanced equation
3. Determine molar ratio of limiting reactant to the product in question
   Ex. C7H6O3 to C9H8O4 is 1:1
4. Determine the maximum amount in mol of product that can be produced based on ratio
5. Convert from moles back to grams for product.

Question 18

What is avogadros law?

Answer 18

Avogadro’s Law states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles. This finding is very important, as it means that we can directly relate the volume of any gas to the amount (in mol) of the gas.

Question 19

What is molar volume of a gas?

Answer 19

The volume occupied by one mole of a gas at conditions of standard temperature and pressure (STP) is the molar volume of a gas.

one mole of a gas at STP occupies a volume of 22.7 dm3 (22700 cm3 or 0.0227 m3)

Question 20

How do you find the moles of a gas

Answer 20

Volume in dm3 divided by molar volume (22.7dm3)

Question 21

What is an ideal gas

Answer 21

An ideal gas is a gas that exhibits the five postulates of the kinetic molecular theory, as well as obeying the gas laws.

Question 22

What is Boyle’s gas law

Answer 22

states that at constant temperature the pressure and volume of a fixed mass of an ideal gas are inversely proportional to each other. if the pressure of a fixed mass of gas at constant temperature is doubled, the volume halves.

PV = k or P ∝ 1/V, where K is a constant, values in Kelvin

Question 23

Outline the five principles of the kinetic molecular theory of matter

Answer 23

1. The particles in a gas are in constant, random, straight-line motion.
2. There are negligible forces of attraction (intermolecular forces) between the particles.
3. Collisions between particles or with the walls of the container are perfectly elastic (no energy is lost).
4. The distance between the particles is much greater than the size of the particles, therefore, gas particles have negligible volume.
5. The kinetic energy of the particles in a gas is directly proportional to the absolute temperature (in kelvin).

Question 24

Charles’ Law

Answer 24

states that at constant pressure the volume of a fixed mass of an ideal gas is directly proportional to its absolute temperature (in kelvin). This means that if the absolute temperature of a fixed mass of an ideal gas is doubled, the volume of the gas will also double.

V ∝ T or V/T = k, where K is a constant

Question 25

What is Gay-Lussac’s Gas Law

Answer 25

states that at constant volume the pressure of a fixed mass of an ideal gas is directly proportional to its absolute temperature (in kelvin). This means that if the absolute temperature of a fixed mass of an ideal gas is doubled, the pressure of the gas will also double.

P ∝ T or P/T = k, where k is a constant 8

Question 26

Combined gas law?

Answer 26

PV / T = k, where k is a constant, P is pressure, V is volume, T is temperature in kelvin.

Question 27

What is the ideal gas equation and explain conversions

Answer 27

PV= nRT
P is the pressure in Pa if kPa are given, multiply by 10^3

V is volume in m3 if dm3 are given, divide by 10^3, or if cm3 are given, divide by 10^6

N is moles
R is universal gas constant

T is temperature in Kelvin if °C is given, add 273

Question 28

Rearrange ideal gas formulae to calculate amount in mol

And rearrange again to calculate molar mass

Answer 28

N = (PV)/RT

Mr= (mRT)/PV

Question 29

What do you call gases that deviate from gas laws and font obey the ideal gas equation

State the two reasons why they don’t obey

Answer 29

Real gases (note: all gases deviate from ideal behaviour to some extent)

At very high pressure the gas particles are closer together. Under these conditions, the actual volume of the particles becomes significant.

At low temperatures, the particles move less rapidly (have lower average kinetic energy). This means that there is a greater opportunity for intermolecular forces between the particles to have an effect.

conditions under which gases can be liquified, which implies that there must be substantial forces of attraction between the particles under these conditions.

Question 30

What is the product for PV/RT for an ideal gas

Answer 30

1

Question 31

What are solutions

Answer 31

Homogenous mixtures, formed when a solute dissolves into a solvent.
They can be dilute or concentrated.

Question 32

How to calculate concentration

Answer 32

Amount of solute (mol) divided by volume of solution in dm3

Question 33

How to calculate concentrations of very dilute solutions like levels of pollution in water

Answer 33

Concentration (ppm) =
(mass of solute, g divided by mass of solution, g) times 10^6

Question 34

What is a titration and how is it performed

Answer 34

method of volumetric analysis in which the concentration of a solution can be determined.

1. a titration, a burette is filled with a standard solution of known concentration (the titrant).
2. A carefully measured volume of the solution with the unknown concentration (the analyte) is placed in a conical flask below the burette.
3. An indicator is used to determine the end-point of the titration.

first trial is known as the rough titration and is not used to calculate the average volume.

Question 35

How to calculate concentration of solution in a titration

Answer 35

1. Calculate mol of the titrant added to the analyte
   using n= cV
2. Determine the molar ratio of the titration to analyte from the equation and the amount in mol of analyte reacting with titration
3. Calculate concentration of the analyte in mol dm-3