# How many grams do 4.3 * 10^21 molecules of UF_6 weigh?

We define a gram-molecular weight as the mass of one mole of any element or compound. A mole contains Avogadro's number of units (molecules) – $6.022 x {10}^{23}$. So, dividing the number of molecules by Avogadro's number gives us the number of moles. Multiplying that by the molecular weight gives us the mass. From the Periodic Table we find the gram-molecular weight of U to be238 and of F to be19. So, $U {F}_{6}$ will have a gram-molecular weight of 352g.