Empirical formula when given percentages?
You will get the empirical formula straight from this data. In order to convert to a molecular formula, you need more information.

By way of another example:


  A binary compound of tin and chlorine contains 62.89% tin by mass. Determine the empirical formula.


Our first goal is to figure out the ratio of moles of tin to chlorine in this compound. Once we know the ratio of moles, we can determine the formula.

What do we know? We know what percentage of tin is in this compound by mass. What is the percentage of chlorine?

In order to get to moles, we need at least a mass of the sample. We were not given that. Since percent by mass is a ratio of masses, it turns out we can choose any mass of sample we want and get the same formula (Try it yourself!).

If we assume that we started with 100.0 g of this stuff, then how many grams of tin would you have? How many grams of chlorine would you have? 100.0 g is a nice starting mass since you are working with percents.

Now, as Ari's answer says, you need to convert mass of tin and mass of chlorine into moles of tin and moles of chlorine by dividing each mass by the atomic molar mass of each element (look it up on your periodic table).

Now that you are in moles, determine the lowest whole number ratio of the two elements by dividing each number of moles by the smaller number of moles. If your numbers are close to whole numbers, you can round. For example, round 1.97 to 2. If your numbers are not close to whole numbers, you may need to do a little more arithmetic:


If you get a number like 1.48, this is close to 1.5 which is 3/2. Multiply both numbers by 3.
If you get numbers like 1.34 (close to 4/3) or 1.67 (close to 5/3) multiply by 3.


Now you have the empirical formula. As Ari's answer says, you need a molar mass of the species to get a molecular formula.

The answer for my example is $\ce{SnCl2}$. Work through the steps and see if you can get to this answer. Then repeat for your lead and oxygen compound.