Unit 3: quantitative chemistry Flashcards
(22 cards)
what is the relative formula mass (Mr) of a compound?
the sum of the relative atomic masses of all the atoms in the compound
what is avogadro’s constant?
6.02 x 10^23
how much will one mole of carbon weigh?
12 g
how do you find the amount of moles in a given mass?
moles of substance = mass/Mr
in a chemical reaction, is mass always lost, gained or conserved?
conserved
why might there appear to be a change in mass in a chemical reaction?
there is a gas involved in the reaction
what is a limiting reactant?
the reactant that will be used up first in a chemical reaction
calculate the mass of aluminium oxide formed when 125g of aluminium is burned in air?
225g of aluminium oxide
how much space will one mole of any gas occupy at 20C?
24dm^3
how do you find the volume of a known mass of any gas at room temperature?
volume of gas= mass of gas/Mr of gas x 24
what is concentration?
the amount of a substance in a certain volume of a solution
how do you calculate concentration?
concentration = mass(or number of moles) of solute/volume of solvent(in dm^3)
a student started with 30cm^3 of sulfuric acid of unknown concentration in a flask. She found that it took an average of 25 cm^3 of 0.1 mol/dm^3 sodium hydroxide to neutralise the sulfuric acid. Find the concentration of the acid in mol/dm^3. The equation is: 2NaOH + H2SO4 = Na2SO4 + 2H2O
0.417 mol/dm^3
how do you convert from cm^3 to dm^3?
divide cm^3 by 1000
What is the atom economy of a reaction?
it is the how much of the mass of the reactants is wasted when manufacturing a chemical and how much mass ends up as a useful product
how do you calculate atom economy?
atom economy= Mr of desired products/Mr of all reactants x 100
calculate the atom economy of the following reaction to produce hydrogen gas. CH4 + H2O = CO + 3H2
17.6%
Why is high atom economy good?
it reduces waste. Reactions with low atom economy use up resources very quickly and create lots of waste that needs to be disposed of somehow, making these reactions unsustainable.
it is cheaper than reactions with low atom economy.
What is percentage yield?
it is a percentage that tells you about the overall success of an experiment. it compares what you think you should get (theoretical yield) with that you get in practice (actual yield)
how do you calculate percentage yield?
percentage yield = mass of product actually made (g)/maximum theoretical mass of product(g) x 100
Why are percentage yields always less than 100%?
1) in reversible reactions, the products can turn back into the reactants, so the yield will never be 100%
2) the reactants might react differently to how you expect. They might react with gases in the air or impurities in the reaction mixture, so they end up forming extra products other than the ones you want
3) you lose some product when you separate it from the reaction mixture or transfer it from one container to another
why do you want to have a high percentage yield?
it reduces waste and costs
