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.
Originally posted by SgStudentStressed: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.
oops i meant X has 4 valence electrons while Y has 6 electrons.
Originally posted by SgStudentStressed:oops i meant X has 4 valence electrons while Y has 6 electrons.
If X is in Grp IV, think Carbon.
If Y is in Grp VI, think Oxygen.
Hence, think Carbon dioxide, which is CO2, and hence you arrive at the answer XY2.
oh so i can use the 'cross method' like in finding ionic eqn? but then im a bit unsure since S and O form SO2 but if i use the method stated ill get SO which is wrong...
Originally posted by SgStudentStressed:oh so i can use the 'cross method' like in finding ionic eqn? but then im a bit unsure since S and O form SO2 but if i use the method stated ill get SO which is wrong...
The so-called 'cross method' is only a guide. You need to base on your familiarity with compounds of period 2 and period 3 elements. For instance, even at O levels, you should already know sulfur dioxide and sulfur trioxide both exist.
so if i use the cross method i will get the it as C2O4 so is that the molecular formula or is that wrong n when i simplify to CO2 then its the molecular formula?
Originally posted by StudentQns:so if i use the cross method i will get the it as C2O4 so is that the molecular formula or is that wrong n when i simplify to CO2 then its the molecular formula?
You should have memorized that the most common oxide of carbon is CO2, so you must give the formula as CO2. The valency (what you call 'cross' method) is just a guide.