Predict the empirical formula of the compound formed by each of the following pairs of elements.a) Li,S b) Na,H c) Al,l d) Ca,O
Look at the periodic table.
Li is in I so it has a valence of +1. S is in group VI (or 16 depending upon the system you are using) and it has a valence of -2; therefore, the formula is Li2S (Note that Li at +1 each is +2 total and that is neutralized by the -2 for S). Compounds must be zero.
Na is in group I BUT it is also in group VII on my chart( meaning it can have a valence of +1, which is the most usual, but it also can have -1 so the formula is NaH. Now you try the others.
To predict the empirical formula of a compound, we need to determine the ratio of elements present in the compound.
a) Li, S:
1. Look up the valence (or oxidation) numbers of the elements. The valence number of Li is +1, and for S it is -2.
2. Determine the least common multiple (LCM) of the absolute values of the valence numbers. The LCM of 1 and 2 is 2.
3. Divide the LCM by the valence numbers to find the ratio of atoms. The ratio is Li2S.
Therefore, the empirical formula for the compound formed by Li and S is Li2S.
b) Na, H:
1. The valence number of Na is +1, and for H it is -1.
2. The LCM of 1 and 1 is 1.
3. Divide the LCM by the valence numbers to find the ratio of atoms. The ratio is Na1H1 (or just NaH).
The empirical formula for the compound formed by Na and H is NaH.
c) Al, I:
1. The valence number of Al is +3, and for I it is -1.
2. The LCM of 3 and 1 is 3.
3. Divide the LCM by the valence numbers to find the ratio of atoms. The ratio is AlI3.
Therefore, the empirical formula for the compound formed by Al and I is AlI3.
d) Ca, O:
1. The valence number of Ca is +2, and for O it is -2.
2. The LCM of 2 and 2 is 2.
3. Divide the LCM by the valence numbers to find the ratio of atoms. The ratio is Ca1O1 (or just CaO).
The empirical formula for the compound formed by Ca and O is CaO.