AP chem lab-finding the % of zinc?

so in my AP chem cl we're doing a lab where we have to find the % of Zn in a sheet of galvanized metal. so everyone cut a small piece of the big sheet of Zn and dumped it in a beaker of HCl. I thought that we have to get the m of sheet before the reaction & m of the sheet after the reaction and compare the two in order to figure out what % of the m was made out of Zn. But my teacher kept on telling us to figure out the VOLUME of the sheet. I don't understand why we can't just calculate it with m. She gave us no written procedures whatsoever and she's horrible at clarifying. Is volume really necessary for this lab? If so, WHY? Also, she told us to divide the volume by 2 because we have "2 sides on the metal sheet" or something... I don't get why we need to do that either. HELP?!