# Tag Info

Molecular mass is the mass of a single molecule Molar mass refers to the mass of 1 mole of molecules, ie $6.023 × 10^{23}$ of them. Eg: Molecular mass of $\ce{H2}$ is about $3.32 \times 10 ^{-24}\ \mathrm g$ or about $2\ \mathrm u$. Here "$\mathrm u$" is just a unit, $1\ \mathrm u = 1.66 \times 10^{-24}\ \mathrm g$ Whereas molar mass is about $2\ \... 3 Let's look at the$\ce{^9_4Be}$isotope and apply the definitions: Relative atomic mass The relative atomic mass of$\ce{^{12}C}$is, by definition, 12. Looking at the periodic table, we find that the relative atomic mass of beryllium is 9.0121831(5). That makes sense because$\ce{^{12}C}$has a mass number of 12 (6 protons and 6 neutrons) while beryllium ... 12 It is the same reason that a dozen doesn't depend on whether you are counting grapes or elephants. That is how the mole is defined: it is a number, nothing else. The confusion, I suspect, is because of how we measure that number (or, strictly, how we originally measured it as the definition changed recently). The intention of the unit was always to define a ... 3 I agree with the starting point of the Matthew's solution, namely the definition of vapor density, but I fail to understand the follow-up math, so I just post my approach. $$M(\ce{ECl_x}) = D\cdot M(\ce{H2}) = 85\cdot\pu{2 g mol-1} = \pu{170 g mol-1}\tag{1}$$ By definition, molar mass$M(\ce{ECl_x})$is also$$M(\ce{ECl_x}) = M(\ce{E}) + x\cdot M(\ce{Cl})$...