Flask contains 0.420 mol of liquid bromine, br2. determine the number of bromine molecules present in the flask. express your answer numerically in molecules.

Since it talks about the number of particles, we would need the Avogadro's number which is an empirical value equal to 6.022×10²³ particles/mole. Therefore, to determine the number of particles, such as molecules, just multiple the number of moles to the Avogadro's number:

Number of Br₂ molecules = 0.42 mol *6.022×10²³ molecules/mole = 2.53×10²³ molecules

My educational guess is B. Hydrogen

Hope that helps! :D