Answer:- 
molecules.
Solution:- The grams of tetrabromomethane are given and it asks to calculate the number of molecules.
It is a two step unit conversion problem. In the first step, grams are converted to moles on dividing the grams by molar mass.
In second step, the moles are converted to molecules on multiplying by Avogadro number.
Molar mass of 
= 12+4(79.9) = 331.6 g per mol
let's make the set up using dimensional analysis:
= molecules
So, there will be molecules in 250 grams of .
Answer:
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Total weight = 1 + 35.45 + 3 * 16 = 84.45
H = 1 / 84.45 * 100% = 1.18%
Cl = 35.45 / 84.45 * 100% = 41.98%
O = 48 / 84.45 * 100% = 56.84%
The answer is HCl, C3H6 and C2H5OH. Hydrochloric acid has the lowest boiling point among the three choices since it only has weak dipole-dipole and Van der Waal's forces between molecules which are much weaker than the forces of attraction present in propane and ethanol. Ethanol has the highest since the hydrogen bonding present in ethanol make it hard to break the bonds.
