And in symbols:
When you make these esters in the labs, often you are astonished by their beautiful aroma. Ethyl butyrate has the odour of fresh pineapple (or maybe fresh pinapples contain this ester; certainly these esters are added to "fresh" packaged fruit juices to enhance their appeal; it really is a beautiful smell).
And, clearly, moles of ester are equivalent to moles of butyric acid.
And thus mass of ester recovered is given by