3: Quantitative Chemistry

Question 1

what is the conservation of mass?

Answer 1

the mass of the reactants always equals the mass of the products - no atoms are lost or made

Question 2

why does it sometimes seem that the mass after a chemical reaction has increased or decreased?

Answer 2

when the system is not closed, mass can seem to change
it can seem to increase if one of the reactants was a gas, which cannot be measured/not taken into account along with the rest of the reactants
it can seem to decrease if one of the products was a gas, which would be lost

Question 3

how do you calculate the percentage of mass of an element in a compound?

Answer 3

% mass of element = Ar x number of atoms of element / Mr of compound x 100

Question 4

what is the value of the Avogadro constant?

Answer 4

6.02 x 10 to the 23rd power per mole

Question 5

equation for mass =

Answer 5

mass = Mr x moles

Question 6

what are systematic errors, how can they be identified and how can they be corrected?

Answer 6

they occur due to a persistent flaw with equipment or incorrect use of equipment and reduce accuracy; can be spotted as there is a trend/pattern in results, but upon analysis, patterns would not be correct; can be corrected by improving experimental technique, by using apparatus with a greater degree of accuracy

Question 7

what are random errors, how can they be identified and how can they be corrected?

Answer 7

they occur due to issues over which the scientist has no direct control - e.g. changes in room temp, and reduce reliability; produces an anomalous result; by using apparatus with a greater degree of accuracy, increasing the number of measurements taken, taking measurements more carefully

Question 8

how do you calculate uncertainty from an instrument?

Answer 8

uncertainty = resolution / 2

Question 9

how do you calculate uncertainty from a set of results?

Answer 9

uncertainty = range / 2
mean = mean value +- uncertainty

Question 10

what is the limiting reagent?

Answer 10

the reactant that is all used up

Question 11

what is the equation for concentration?

Answer 11

concentration = moles or grams / volume (dm cubed)

Question 12

how do you convert a concentration from g/dm cubed to mol/dm cubed?

Answer 12

divide by Mr

Question 13

why do we never get 100% yield?

Answer 13

some reactants may be left behind in the equipment; products can be lost during transfer from container to container, or during separation stages e.g. filtration, distillation; if reaction is reversible, the products are continually turning back into the reactants - reaction may not go to completion; some of the reactants may react in ways different to the expected reaction

Question 14

how do you calculate percentage yield?

Answer 14

% yield = actual mass made / theoretical max. mass x 100

Question 15

what is atom economy and why is it important?

Answer 15

its a measure of the amount of starting materials that end up as useful products; important for sustainable development and for economic reasons to use
reactions with high atom economy

Question 16

how do you calculate the percentage atom economy?

Answer 16

Mr of desired product / sum of Mr of all reactants x 100

Question 17

why is a low atom economy bad?

Answer 17

a lot of resources are used up (raw materials can be expensive); a lot of waste material is produce - very expensive procedure, requires chemicals, equipment, space and transport
unsustainable as they use up too much raw material to manufacture only a small amount of product

Question 18

how can we improve atom economy?

Answer 18

sell or reuse waste products; alt methods of production could be considered - produce more useful by-product

Question 19

describe how to do a titration.

Answer 19

1. use the pipette & pipette filler to add a measured volume of sodium hydroxide solution to a clean conical flask.
2. add a few drops of indicator (e.g. methyl orange) and put the conical flask on a white tile.
3. fill the burette w hydrochloric acid & note the starting volume
4. slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
5. top adding the acid when the end-point is reached (when the indicator first permanently changes colour). 6. note the final volume reading.
6. repeat steps 1-5 until concordant titres are obtained. More accurate results are obtained if acid is added drop by drop near to the end-point.

Question 20

why is a pipette used to measure the alkali, but a burette is used to measure the acid?

Answer 20

pipette - measures fixed volume; burette - variable volumes

Question 21

what is Avogadro’s Law?

Answer 21

at the same temp. & pressure, equal amounts of gases occupy the same volume of space

Question 22

what is the volume of one mole of gas at RTP (room temp and pressure)?

Answer 22

24 dm cubed

Question 23

how do you calculate the volume of gas at RTP

Answer 23

volume (dm cubed) = moles x 24