PCl₅, BF₃, and C₆H₁₂O₆ are not ionic compounds, and can thus be eliminated because "formula weight" implies that we are dealing with an ionic compound. This leaves only the correct answer, KCl, which can also be verified as correct by calculating the weight. K is 39g, while Cl is 35.5g. Together, this gives a total weight of approximately 74.5g.

An empirical formula is a way of expressing a chemical formula in the smallest possible ratio. The compound C₃H₆ has an empirical formula of CH₂ because the simplest ratio between this compound's constituent elements is one carbon for every two hydrogens.  This empirical formula can represent a number of compounds, but we know that each compound has a molar mass that is a multiple of the molar mass of the empirical formula. The empirical formula has a molar mass of 14 g/mol so the molar masses of compounds that can be represented by this empirical formula are (28,42,56 . . .).  

Lithium iodate is comprised of and . The anion refers to the negative ion, so we are looking for the percent by mass of .

The total molar mass of lithium iodate is .

The anion, , contributes 190.9g to the total mass.

Find the percent by mass by dividing the mass of the anion by the total molecular mass.

To find the molecular weight, you need to add together the atomic masses of each of the atoms. 1 sulfur is 32g/mol, 1 oxygen is 16g/mol, and 2 chlorine is 71g/mol (each Cl atom is 35.5g/mol). These values are taken from the periodic table. Added together, they make 119g/mol.

The chemical formula  has a 2 right outside of the parentheses surrounding nitrate, so we need to distribute/multiply that 2 to everything inside the parentheses. That means that we have 1 calcium, two nitrogens, and six oxygens, and we need to add up all those masses to get the molar weight.

In order to find the molar mass of potassium sulfide, you must first write out the chemical formula for potassium sulfide. The symbol for the potassium ion is , and the symbol for sulfur is . We know that this is an ionic compound because we have a metal involved (potassium) with a nonmetal (sulfur), so we must balance the net charge while making sure both atoms have a full octet. Since potassium is in group I, we know that it can easily lose one electron to fill its outer shell of electrons. Sulfur is in group VI, so we know that it can easily gain two electrons to fill its valence shell of electrons. It would take two potassium atoms to donate one electron each to sulfur to make this compound stable. Thus the chemical formula would be . Use the periodic table to find the molar mass.

The formula for ammonium hydroxide is . To find molar mass, add up the individual masses of each element, and remember to multiply that mass by the molar ratio of that element.

First, write the formula for oxalic acid: . We know that this formula is correct because when we have an acid with an "-ic" ending and no "hydro-" prefix, that means it's coming from a polyatomic ion with an "-ate" ending. So we know that oxal"ic" acid is derived from the oxal"ate" polyatomic ion. The formula for that ion is . Putting a hydrogen atom in front of this ion, and balancing the charges, we get .Then add the atomic masses of each of the individual elements in their respective ratios, to get the molar mass for the whole compound.

First, find how many moles of chlorine are present in the sample:

Because the ratio of moles of  to moles of  is 1:1, there are also 2 moles of . The next step is to find the mass of 2 moles of .

Find the number of moles of each element. 

If the chemist has 2.0 moles of the compound, 1 mole would contain half as much. So 1mol of carbon, 4mol of hydrogen, and 2mol of oxygen, which forms acetic acid