Topic 3: Quantitative Chemistry Flashcards
What is the law of
conservation of mass?
The law of conservation of mass states that no atoms are lost or made
during a chemical reaction so the mass of the products equals the mass
of the reactants.
Define relative atomic mass
the average mass of atoms in an element taking into account masses
and abundance of its isotopes,
what is the sign for relative atomic mass
Ar
Define relative formula
mass.
Relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
what is the sign for relative formula mass
Mr
key facts about relative formula mass
- has no units
- never involves big numbers
why might some reactions appear to show a change in mass?
if the mass appears to change this is because of a gas. if one reactant or product is a gas and if there is a change in the mass this is because the gas has escaped
Give two examples of a reaction where a change in mass may appear to take place.
Metal reacting with oxygen or an acid. Thermal decomposition.
how would you solve this problem
you could use a sealed container so that it would trap the gas
Why does the water, produced during the reaction, have a lower mass than the original hydrogen peroxide?
Because the oxygen gas produced during the reaction escaped into the atmosphere.
Chemical measurements
Whenever a measurement is made there is always some uncertainty about the result obtained
The symbol for the unit mole
mol
number of molecules in one mole =
6.02 x 10 ^23
number of moles =
mass (g) / relative atomic mass AR
or mass / relative formula mass
mass =
number of moles x Ar
or
number of moles x Mr
What is the relative formula?
mass of:
A) CaF2
B) B) C6H12O6
CaF2
- (Ar
values: Ca = 40, F = 19)
40 + 19 + 19 = 78
C2H12O6
- (Ar
values: C = 12, H = 1, O = 16)
(12 x 6) + (1 x 12) + (16 x 6) = 180
What is Avogadro’s
constant?
The number of atoms, molecules or ions in a mole of a given substance.
The value of the constant is 6.02 x 1023
What is the formula that
links mass, molecular mass
and moles together
Mass = Mr x Moles
What is the mass of:
20 moles of calcium
carbonate, CaCO3
Mass = Mr x Moles
Mr = 100
100 x 20 = 200 g
Calculate the amount of carbon dioxide
in moles in 0.32 g of carbon dioxide.
Relative atomic masses (A
r): carbon =
12, oxygen = 16
Moles = Mass / Mr
0.32 / 44 = 0.007
moles=
mass/RAM
State what we mean by a
limiting reactant in a
chemical reaction
In a chemical reaction involving two reactants, it is common to use an
excess of one of the reactants to ensure that all of the other reactants is
used. The reactant that is completely used up is called the limiting
reactant because it limits the amount of products.
unit of concentration
grams per dm3
g/dm3
the equation for concentration =
mass (g) / volume (dm^3)
the triangle formula
m ass
c oncentration x v olume
higher concentration…
if we keep the volume the same and increase the mass of the solute it gives a higher concentration
lower concentration …
if we increase the volume of the solution and keep the mass the same it gives a lower concentration
Write down the two formulae that
link concentration, mass and
volume together.
Concentration (g per dm3) = Mass (g)/Volume (dm3)
Concentration (mol per dm3) = nr of moles/volume (dm3
Explain how the mass of a solute and the volume of water effect the concentration of a solution
A larger mass of solute in a certain volume of water → more concentrated solution, [1]
the larger volume of water for a certain mass of solute → less concentrated solution [1]
why does the mass decrease when metal reacts with oxygen
The mass of the oxide produced will be larger than the mass of the starting metals because oxygen atoms from the atmosphere have been added
what happens in the reaction of Zinc + hydrochloric acid
During this reaction, you would expect the mass of the flask and reactants (zinc and hydrochloric acid) to decrease.
This is because the hydrogen gas produced will escape from the flask.
