This appears to be a homework question. So, I will not give you the answer. However, I will tell you how to get the answer. This is how you figure out the answer to this question. First of all, note that you are given the molar mass of the oxide. So, let us assume that we have 1 mol of the oxide. That means that we have 229 grams of it. Now realize that manganese constitutes 72.0% of this mass. So, use this to calculate how many grams of manganese you have. (In other words, what is 72.0% of 229g?) Subtract this value from the total mass to find out how much of it is oxygen. Once you have done this, you will know how much Mn and O you have by mass. Now you want to go to your Periodic Table of the Elements and look up the molar mass (atomic weights) of each of these elements. You divide the mass of each element that you just figured out by the corresponding molar masses. This will tell you how many moles of each you have in your sample. If these numbers are whole numbers, then you look at them to see if they have any common factors to see if you can simplify the ratio. For example let us assume that you found that you had 18 mols of Mn and 21 mols of O. You can look at both of the numbers and see that both can be divided by 3. So, the Chemical formula would be Mn6O7. (Note, I have not actually done the calculations here so, this example can't be assumed to be the correct answer to your question. In fact, you can be pretty sure that it is not the right one.) Ok, now you have the procedure to answer this question. If you will post an answer as a comment to this question, then I will tell you if you are right.

275 mL?