for ionic compounds the empirical formula and molecular formula
is the same right? eg NaCl is the empirical and molecular. MgO is
the empirical and also the molecular formula.
but covalent substance's empirical and molecular formula can
either be the same or different rite? eg H2O2 is the molecular bt
empirical is HO whiel NH3 is both the empirical formula n the
molecular formula. so how do i determine a covelant substance's
molecular formula? also why does NH3 have an empirical formula
which is the same as its molecular formula while H2O2 has an
empirical formula which is different from its molecular
formula?
eg. this came out in my EOY examination. X has a valency of 4
while Y has a valency of 2. X has an Ar of 10 while Y has an Ar of
15. (dummy values) so we know that it can either be an ionic or
covalent substance here. so the empirocal formula will be XY2. if
it is a covalent substance then it can have many possible molecular
formulas, bt then how do we tell what is the molecular formula?
also when they call us to find the Mr of a compound/molecule do we
take the empirical or molecular forumla? cos if we just take the
empirical formula then it would be simple just 10+15+15=40. but if
we takethe molecular formula then it get complicated due to
my reasoning.
thanks for helping me with this.
If a compound is ionic, it does not contain any molecules,
so it does not have a molecular formula.
For Mr (which stands for "relative formula mass" if it's an
ionic compound, or "relative molecular mass" if it's a covalent
compound) or molar mass, of a molecular compound whose empirical
and molecular formulae differ, the exam-smart thing to do, would be
to give BOTH Mr values, but clearly labelled :
Example : Hydrogen peroxide :
Mr based on molecular formula H2O2 = 34
Mr based on empirical formula HO = 17
But generally and by default, you should always be giving the Mr
based on the molecular formula (as long as the molecular formula
differs from the empirical formula).
As for your exam question, as you say, an element with a
valency of 2 could either be a Grp II metal, or a Grp VI non-metal.
As to giving the possible formulae for such compounds, you should
base it on a real-life example you're familiar with.
Eg. If the element is a Grp VI non-metal, it could be oxygen.
And since the other element is in Grp IV, it could be carbon.
Therefore, give the formula of the most common species for such a
compound, ie. CO2 (rather than CO, whose structure and bonding is
more complicated than O levels can handle).
Don't worry so much about all the other possible molecular
formulae, if it's a covalent compound. Just give the most common
formula, based on real-life examples.
And for O levels, base your answers on period 2
elements, rather than period 3 elements. Since in period 3, there
will be more possibilities to consider, eg. PCl3, PCl5, etc.