1.2 Amounts of substances

Question 1

What is the definition of Relative Atomic Mass?

Answer 1

average mass of an atom of an element

on a scale where an atom of carbon-12 has a mass of 12

Question 2

Relative Molecular Mass

Answer 2

Average mass of a molecule on a scale where an atom of carbon-12 has a mass of 12

Question 3

Diff between Relative Formula mass and Mr

Answer 3

For ionic compounds (that do not exist as molecules) the term relative formula mass is used instead.

Question 4

Formula of Complex Ion Manganate (VII)

Answer 4

MnO4-

Question 5

Complex Ion Chromate

Answer 5

CrO2^-

Question 6

Complex Ion Dichromate

Answer 6

Cr2O7^2-

Question 7

Why are ionic compounds in Relative Formula Mass?

Answer 7

Because ionic compounds don’t really form molecules (they form large lattices) we cannot talk about molecular units.

Question 8

Chromium (III) sulphate formula

Answer 8

Cr2(SO4)3

Question 9

Ammonium nitrate

Answer 9

NH4NO3

Question 10

Copper (II) sulphide

Answer 10

CuS3

Question 11

Diff btw sulphide and phate

Answer 11

Ide- just ion with oxygen

ate- with oxygen SO4

Question 12

Deduce the formula of the compound that contains 2+ ions and 3− ions that both have the same electron configuration as argon.

Answer 12

Originally had two more electrons than Ar, 2+ ion = Ca2+
Originally had three less than Ar, 3- ion = P3-
Ca3P2

Question 13

Significant figures for calculations involving

multiplication / division

Answer 13

Your final answer should be given to the same number of significant figures as the least number of significant figures in the data used.

Question 14

Significant figures for calculations involving

addition/subtraction ONLY

Answer 14

Here the number of significant figures is irrelevant – it is about the place value of the data. We round our answer to the data with the fewest number of decimal places.
answer cannot have more decimal places than
either of the original numbers

Question 15

SI Units

Answer 15

Amperes, Kelvin (temp), Kilograms (mass), Candela (light), Moles, Seconds (time), Metres

Question 16

Calculate Mr of aspirin (search up skeletal structure)

Answer 16

C9H804

Question 17

What are the two formulas for moles that link?

Answer 17

moles = mass (g)/ Mr

moles in solution =
concentration c (mol dm-3) x volume V (cm^3)

Question 18

One mole of any substance contains the same amount of particles…

Answer 18

6.022 x 10^23

Question 19

What is the limiting factor?

Answer 19

In the real world of chemistry, it is rare that we react the exact right amount of chemicals together. Usually, we have more than we need of one of the reactants and so it doesn’t all react – it is in excess.

Sometimes in calculations, we need to work out if one of the reactants is in excess. The reactant that is not in excess is sometimes called the limiting reagent.

Basically- reactant that is used up the fastest and produces the least amount of product- reactant not in excess

Question 20

Three step to work out limiting factor and do example from white cards

Answer 20

1) Find masses and Mrs
2) Mol for one reactant
3) Look at big no. to find theoretical moles for other reactacnt- multiply moles by big no.
4) Find actual moles for reactant
5) Compare- if actual is less than theoretical then reactant is limiting
6) Limiting (actual) moles x big no. in front of product is product moles

Question 21

Steps to work out the empirical formula

Answer 21

Write the % or mass of each element

Divide each % or mass by that element’s Ar

Divide each answer by the smallest answer from above to find the simplest ratio

Def:The lowest whole number ratio of atoms of elements in a compound

Molecular formula= molar/empirical
Then multiply answer by empirical

Divide the actual molar mass by the mass of one mole of the empirical formula.

Question 22

Caffeine has a molar mass of 194 g. what is its molecular formula? Empirical formula = C4H5N2O1

Answer 22

molecular formula
= molar mass/ empirical formula mass
194/97=2

2 x C4H5N2O1= C8H10N402

Question 23

What do the gas laws look at relationships between?

Answer 23

Temp, pressure and volume

Question 24

Boyle’s Law

Answer 24

Boyle’s law -describes the relationship between PRESSURE and VOLUME.

The product of pressure multiplied by the volume of a gas in an enclosed container remains the same if either the pressure or volume changes and the temperature remains constant so…

P↑, V↓ and vice versa

If you double the pressure, vol of gas halves

PV= constant

Question 25

Charles' Law

Answer 25

TEMP and VOLUME If you DOUBLE the absolute temperature of a gas, the volume DOUBLES. T↑V↑ V/T= constant

Question 26

Gay-Lussac's law

Answer 26

Relationship between TEMP and PRESSURE The pressure of a gas VARIES DIRECTLY with the absolute temperature of the gas when volume remains constant. (e.g think of empty deodorant can- is cold when empty) T↑P↑ P/T= constant

Question 27

Combined Gas Laws- equation, what constant depends on, units etc

Answer 27

PV/T= constant constant depends on the conditions used e.g- temp, press, moles of gas If we have one mole of gas at a standard temp + press we can call this the gas CONSTANT represented by R PV= nRT R (given in Q)= 8.31 JK -1 mol-1 P= Pascals (Nm-1) V=m^3 T=K

Question 28

Ideal gas equation

Answer 28

PV= nRT ``` R (given in Q)= 8.31 JK -1 mol-1 P= Pascals (Nm-1) V=m^3 T=K n= no. of moles ```

Question 29

Ideal Gas Def

Answer 29

In an ideal gas… 1. Collisions are perfectly elastic 2. No intermolecular attractive forces (i.e all internal energy = kinetic energy) 3. Therefore does not condense into a liquid when cooled. In reality, there are no gases that fit this definition perfectly. We assume that gases are ideal to simplify our calculations.

Question 30

Molar Volume def, how it can be calculated, rtp, equation, what it is at rtp

Answer 30

The volume occupied by 1 mole of a gaseous substance is called its molar volume. Molar volume can be calculated from the density of the gas and its formula mass, Mr. Since volume depends on both temperature and pressure, molar volume is generally measured at “room temperature and pressure”, rtp, which is about 20oC and 1 atmosphere. DMV- Density = mass/volume rearranging this we get…. Molar volume = mass of 1 mole (Mr) /density at rtp. The molar volume of any gas at rtp is approximately 24 dm3. Or, since there are 1000 cm3 in 1 dm3 The molar volume is also 24000 cm3.

Question 31

What can we calc from a balanced equation

Answer 31

How much of any given product can be made. In the real world of industry there are 2 further important factors 1. The yield of a reaction. (Usually expressed as a %) 2. Atom economy. This is a theoretical value.

Question 32

Why might the theoretical yield fall short? What is the theoreyical yield equation?

Answer 32

Not all product is recovered (e.g. spattering, loss) Reactant impurities (e.g. weigh out 100 g of chemical which has 20 g of junk) A side reaction occurs The reaction does not go to completion Inaccuracy in measurement (no. of moles of product/theoretical max no. of moles of product) x100 (actual/theoretical)x 100

Question 33

What is the % yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2?

Answer 33

Step 1: write the balanced chemical equation N2 + 3H2 → 2NH3 ``` Step 2: Calculate theoretical yield (actual is given): 1 mol H2 ≡ 2/3 mol NH3 20.0 mol H2 ≡ 13.3 mol NH3 Mass of NH3 13.3 x 17.0=227 ``` Step 3: Calculate % yield act/theo (40.5/227)x100=17.87%

Question 34

cm3 to dm3 to m3

Answer 34

x1000→x1000→ /1000←/1000←

Question 35

Two moles calcs (one spec for titrations)

Answer 35

``` mol/mass = conc x vol mol= mass/Mr ```

Question 36

What does M mean in terms of conc?

Answer 36

Concentration is usually measured in mol/dm3 (moldm-3) Sometimes concentration is referred to as ‘molar’ i.e 2M NaOH This means the same as 2 moldm-3 of NaOH

Question 37

You have an unknown acid. How can you determine the concentration of this unknown acid?

Answer 37

We can use an alkali to react with the acid. When the acid has been neutralised, the amount of alkali added allows us to determine the concentration of acid.

Question 38

You have an unknown concentration of HCl acid. 25.0cm3 of this acid is neutralised by 30.0cm3 of 0.10M NaOH. What is the concentration of acid?

Answer 38

1) Balanced eq NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l) 2)moles of NaOH (known conc) Moles NaOH = 0.10 x 30/1000 = 3.0 x 10-3 mol 3) Balanced equation: check mole ratio: NaOH : HCl is 1:1 4) Amount (moles) HCl) Moles = 3.0 x 10-3 5) Concentration of HCl Conc. = 3.0 x 10-3/0.025= 0.12moldm-3