Topic 5 - Amount of Substance

Question 1

Avogadro’s constant

Answer 1

6.02 × 10^23 mol^-1

Question 2

Molar mass

Answer 2

The mass per mole of the substance in g mol^-1

Question 3

Empirical formula

Answer 3

the simplest whole number ratio of the elements present in one molecule or formula unit of the compound

Question 4

Molecular formula

Answer 4

the actual number of atoms of each element in a molecule of the compound

Question 5

Accuracy

Answer 5

How close a measured quantity is to the true value.

The true value is often unknown in chemical analysis, so estimations of the measurement uncertainty is needed.

Question 6

Acid-base indicator

Answer 6

substances used to show changes in pH of solutions and detect end-points in titrations (e.g. phenolphthalein, methyl orange, bromothymol blue)

Question 7

Atom economy

Answer 7

a measure of how efficiently a chemical reaction converts the atoms in its reactants to atoms in the product

Question 8

Atom economy equation

Answer 8

(Mr of atoms in product/sum of Mr of all products) × 100%

Question 9

Bias

Answer 9

the systematic deviation of laboratory test results from the actual value

arises from systematic errors, which affect all measurements in the same way and do not average out

Question 10

Concentration

Answer 10

how much solute is dissolved in a certain volume of solution (g/dm^3 or mol/dm^3)

Question 11

Concentration/g dm^-3 equation

Answer 11

mass of solute (g)/volume of solution (dm^3)

Question 12

Concentration/mol dm^-3 equation

Answer 12

amount of solute (mol)/volume of solution (dm^3)

Question 13

Dilution

Answer 13

the process of adding more solvent to a solution to lower the concentration

Question 14

End point

Answer 14

the point at which a colour change shows that enough of the solution in the burette has been added to react with amount of chemical in the flask

Question 15

Equivalence point

Answer 15

reached when the amount of reactant added from a burette is just enough to react exactly with all the measured amount of chemical in the flask as shown by the balanced equation

Question 16

Measurement uncertainty

Answer 16

unavoidable differences between measured values and true values arising from random and systematic errors of measurement

Question 17

Ideal gas equation

Answer 17

pV = nRT

p = N/m^2 or Pa
V = m^3
T = K
n = moles
R = gas constant (J K^-1 mol^-1)

Question 18

Limiting reactant

Answer 18

a substance which is present in an amount which limits the theoretical yield

Question 19

Molar volume

Answer 19

the volume of one mole of the gas molecules under specified conditions

Question 20

Percentage composition

Answer 20

the percentage by mass of each of the elements in a pure sample of a compound

Question 21

Precise

Answer 21

when repeat measurements have values which are close to each other. Precise measurements have a small random error

Question 22

Pressure

Answer 22

force per unit area in Pa
1 Pa = 1 N/m^2

Question 23

Primary standard

Answer 23

a chemical which can be weighed out accurately to make up a standard solution

Question 24

Relative atomic mass (Ar)

Answer 24

the mean mass of the atoms of an element relative to 1/12 of the mass of an atom of the isotope carbon-12

Question 25

Relative formula mass (Mr)

Answer 25

the sum of the relative atomic masses of all the atoms in a substance's formula

Question 26

Relative molecular mass

Answer 26

the sum of the relative atomic masses of all the atoms in a substance's molecular formula (formula mass but for molecules)

Question 27

Standard solution

Answer 27

a solution with an accurately known concentration

Question 28

Strong acids

Answer 28

acids that are fully ionised when they dissolve in water

Question 29

Titration

Answer 29

a volumetric analysis technique for finding the concentrations of solutions and to investigate the amounts of chemicals involved in reactions

Question 30

Volume

Answer 30

the amount of space taken up by a sample (m^3)

Question 31

Weak acid

Answer 31

acids that are only slightly ionised when they dissolve in water organic acids e.g. ethanoic acid, citric acid inorganic acids nitrous acid, chloric (I) acid

Question 32

% yield equation

Answer 32

actual yield/theoretical yield × 100

Question 33

Writing ionic equations

Answer 33

- write out the full equation for the reaction - replace the formulae of ionic compounds by their separate ions - remove spectator ions

Question 34

Writing ionic half equations

Answer 34

- make the electrons balance - combine the equations - cancel electrons on each side out

Question 35

Acid + metal

Answer 35

-> salt + hydrogen

Question 36

Acid + base

Answer 36

-> salt + water

Question 37

Acid + alkali

Answer 37

-> salt + water

Question 38

Acid + metal oxide

Answer 38

-> salt + water

Question 39

Acid + metal hydroxide

Answer 39

-> salt + water

Question 40

Acid + metal carbonate

Answer 40

-> salt + water + carbon dioxide

Question 41

Acid + metal hydrogencarbonate

Answer 41

-> salt + water + carbon dioxide

Question 42

Amount of substance

Answer 42

the means of counting atoms symbol: n unit: mol

Question 43

Mole

Answer 43

the amount of substance that contains the same number of particles as there are atoms in 12.0 g of carbon-12

Question 44

Ideal gases

Answer 44

gases which obey the gas laws perfectly

Question 45

Avogadro's law

Answer 45

equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

Question 46

mol, vol, 24 equation

Answer 46

mol = vol (dm^3)/24 or mol = vol (cm^3)/24,000

Question 47

Concentration equations

Answer 47

mol. = conc. × vol. (dm^3) mol. = (conc. × vol. (cm^3))/1000 conc. (mol/dm^3) = amount of solute (mol)/vol. of solution (dm^3) conc. (g/dm^3) = mass of solute (g)/vol. of solution (dm^3)

Question 48

Preparing a standard solution

Answer 48

- dissolve a weighted sample of a primary standard in water - mark the solution up to a definite volume in a graduated flask

Question 49

reasons for carrying out dilutions

Answer 49

- to make a solution with the concentration needed for a particular experiment from a standard solution - to dilute an unknown sample for analysis - to give a concentration suitable for titration

Question 50

diluting a solution (method)

Answer 50

- take a measured volume of the more concentrated solution with a pipette and run it into a graduated flask - fill the flask to the mark with purified water

Question 51

concentration volume equation

Answer 51

CaVa = CbVb

Question 52

Phenolphthalein colour change

Answer 52

- bright pink in alkaline solutions - colourless in acidic/near-neutral solutions

Question 53

Methyl orange colour change

Answer 53

- yellow in alkaline solutions - orange in neutral solutions - red in acidic solutions