Physical - Amount of Substance

Question 1

What is the rule with significant figures?

Answer 1

With significant figures, your final answer should be given to the same number of significant figures as the least number of significant numbers used in the data given.

Question 2

What is the rule with decimal places?

Answer 2

With decimal places, we round our answer to the data with the fewest number of decimal places.

Question 3

What are SI Units?

Answer 3

• International system of units.
• There are 7 base units from which all other units are derived.
• There are 20 prefixes which show multiples or
  fractions of the base quantity.
• The base units are derived from invariable constants in nature, e.g. the speed of light.
• Adopted by all countries except the US, Liberia and Myanmar.

Question 4

What is the relative atomic mass?

Answer 4

The relative atomic mass Ar is the weighted average mass of an atom of an element, taking into account its naturally occurring isotopes, relative to 1/12th the relative atomic mass of an atom of carbon-12.

average mass of one atom of an element / (mass of one atom of carbon-12 x 1/12)

Question 5

What is the relative molecular mass?

Answer 5

The relative molecular mass Mr of a molecule is the mass of that molecule compared to 1/12 the relative atomic mass of an atom of carbon-12.

Question 6

What is the Avogadro constant?

Answer 6

The Avogadro constant or Avogadro number is the number of atoms in 12g of carbon-12.

6.022x10^23

Question 7

What is the mole?

Answer 7

The amount of substance that contains 6.022x10^23 particles is called a mole.

The relative atomic mass of any element in grams contains one mole of atoms. The relative molecular mass (or relative formula mass) of a substance in grams contains one mole of entities. You can also have a mole of ions or electrons.

Question 8

How do you work out the number of moles?

Answer 8

mass / Mr

Question 9

What is the relative formula mass?

Answer 9

The relative molecular mass for ionic compounds.

Question 10

Why is the volume of a given mass of any gas not fixed?

Answer 10

It changes with pressure and temperature. However, there are a number of simple relationships for a given mass of gas that connect the pressure, temperature, and volume of a gas.

Question 11

What is Boyle’s law?

Answer 11

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.

Pressure and volume are inversely proportional.

pressure x volume = constant

Question 12

What is Charles’ law?

Answer 12

The volume is directly proportional to the temperature as long as the pressure remains constant.

volume / temperature = constant

Question 13

What is Gay-Lussac’s law? (the constant volume law)

Answer 13

The pressure of a gas varies directly with the absolute temperature of the gas when volume remains constant.

pressure / temperature = constant

Question 14

What equation does combining these relationships give?

Answer 14

(pressure x volume) / temperature = constant for a fixed mass of gas

Question 15

What is the ideal gas equation?

Answer 15

In one mole of gas, the constant is given the symbol R and is called the gas constant. For n moles of gas:

pressure P (Pa) x volume V (m3) = number of moles n x gas constant R (J K-1 mol-1) x temperature T (K)

Question 16

What is the value of R?

Answer 16

8.31 JK-1 mol-1

Question 17

Why is it called the ideal gas equation?

Answer 17

No gases obey it exactly, but at room temperature and pressure it hold quite well for many gases. It is often useful to imagine a gas which obeys the equation perfectly - an ideal gas.

Question 18

What is the empirical formula?

Answer 18

The empirical formula is the formula that represents the simplest whole number ratio of the atoms of each element present in a compound.

Question 19

How do you find the empirical formula?

Answer 19

1. Find the masses of each of the elements present in a compound (by experiment).
2. Work out the number of moles of atoms of each element.
   number of moles = mass of element / mass of 1 mol of element
3. Convert the number of moles of each element into a whole number ratio by dividing each answer by the smallest answer.

Question 20

What is the molecular formula?

Answer 20

The molecular formula gives the actual number of atoms of each element in one molecule of the compound (it applies only to substances that exist as molecules).

Question 21

How do you find the molecular formula?

Answer 21

To find the number of units of the empirical formula in the molecular formula, divide the relative molecular mass by the relative mass of the empirical formula.

Question 22

What is an ideal gas?

Answer 22

In an ideal gas:

• collisions are perfectly elastic
• no intermolecular attractive forces (i.e. all internal energy = kinetic energy)
• therefore does not condense into a liquid when cooled

Question 23

What is molar volume?

Answer 23

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”, which is about 20 degrees Celsius and 1 atmosphere.

Question 24

What is the molar volume equation?

Answer 24

molar volume = mass of 1 mole / density at rtp

Question 25

What is the molar volume of any gas at rtp?

Answer 25

24dm3

Question 26

What is combustion analysis?

Answer 26

Organic compounds are based on carbon and hydrogen. One method of finding empirical formulae of new compounds is called combustion analysis. It is used routinely in the pharmaceutical industry. It involves burning the unknown compound in excess oxygen and measuring the amounts of water, carbon dioxide, and other oxides that are produced. The gases are carried through the instrument by a stream of helium.

Question 27

What is stoichiometry?

Answer 27

The ratio in which the reactants react and the products are produced, in simple whole numbers, is called the stoichiometry of the reaction. You can build up a stoichiometric relationship from experimental data by working out the number of moles that react together. This leads to a balanced symbol equation.

Question 28

What are spectator ions?

Answer 28

ions that do not take part in the reaction

Question 29

What is atom economy?

Answer 29

mass of desired product / total mass of reactants

Question 30

What is the difference between atom economy and the yield?

Answer 30

The atom economy tells us in theory how many atoms must be wasted in a reaction. The yield tells us about the practical efficiency of the process.

Question 31

How may reactants be lost?

Answer 31

- the practical process of obtaining a product (not all product is recovered) - as a result of reactions that do not go to completion - reaction impurities - a side reaction occurs - inaccuracy in measurement

Question 32

What is the percentage yield?

Answer 32

(actual yield / theoretical yield) x 100

Question 33

Why is it desirable to have a high atom economy?

Answer 33

In industrial chemical processes, it is desirable to have a high atom economy for a reaction. This means there is little or no waste product, only the desired product. Therefore it means the process is more economically viable for industrial scale manufacture. Having a high atom economy is also beneficial for the environment as it uses less natural resources and therefore often uses less energy.

Question 34

What are the steps for titration?

Answer 34

1. Rinse out pipette with the solution you are about to use. 2. Fill the pipette with the unknown solution. 3. Initially take more than required. 4. Carefully release the solution until the bottom of the meniscus is touching the line. 5. Transfer to a clean conical flask. 6. The pipette is calibrated, so touch the pipette to liquid in flask to remove all solution (not blow!). 7. The amount in the pipette is called the aliquot (usually 25cm3). 8. Fill Burette at eye level with your known solution. 9. Check the tap is closed! Fill using a funnel, tilted slightly. Add 2 or 3 drops of indicator. 10. Carry out a titration (rough). 11. Remember to swirl the flask continuously. 12. Stop when permanent colour change is observed (the end point). 13. For the accurate run, 5cm3 before the end, start adding solution dropwise. Repeat until successive titres are within 0.1 cm3. 14. Titres that are within 0.1cm3 are called concordant.