3: Quantitative Chemistry Flashcards
what is the conservation of mass?
the mass of the reactants always equals the mass of the products - no atoms are lost or made
why does it sometimes seem that the mass after a chemical reaction has increased or decreased?
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
how do you calculate the percentage of mass of an element in a compound?
% mass of element = Ar x number of atoms of element / Mr of compound x 100
what is the value of the Avogadro constant?
6.02 x 10 to the 23rd power per mole
equation for mass =
mass = Mr x moles
what are systematic errors, how can they be identified and how can they be corrected?
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
what are random errors, how can they be identified and how can they be corrected?
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
how do you calculate uncertainty from an instrument?
uncertainty = resolution / 2
how do you calculate uncertainty from a set of results?
uncertainty = range / 2 mean = mean value +- uncertainty
what is the limiting reagent?
the reactant that is all used up
what is the equation for concentration?
concentration = moles or grams / volume (dm cubed)
how do you convert a concentration from g/dm cubed to mol/dm cubed?
divide by Mr
why do we never get 100% yield?
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
how do you calculate percentage yield?
% yield = actual mass made / theoretical max. mass x 100
what is atom economy and why is it important?
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
how do you calculate the percentage atom economy?
Mr of desired product / sum of Mr of all reactants x 100
why is a low atom economy bad?
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
how can we improve atom economy?
sell or reuse waste products; alt methods of production could be considered - produce more useful by-product
describe how to do a titration.
- use the pipette & pipette filler to add a measured volume of sodium hydroxide solution to a clean conical flask.
- add a few drops of indicator (e.g. methyl orange) and put the conical flask on a white tile.
- fill the burette w hydrochloric acid & note the starting volume
- slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
- top adding the acid when the end-point is reached (when the indicator first permanently changes colour). 6. note the final volume reading.
- 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.
why is a pipette used to measure the alkali, but a burette is used to measure the acid?
pipette - measures fixed volume; burette - variable volumes
what is Avogadro’s Law?
at the same temp. & pressure, equal amounts of gases occupy the same volume of space
what is the volume of one mole of gas at RTP (room temp and pressure)?
24 dm cubed
how do you calculate the volume of gas at RTP
volume (dm cubed) = moles x 24
