Chemblog!

Here is a hint related to what I spoke about in class; this question isn't as difficult as it seems.
Recall our simple stoichiometry calculations of empirical formulas from the percent composition of each element in a compound.
For example, water is 88.9 % oxygen by mass and 11.1% hydrogen by mass. IF WE KNEW the molar mass of water (from gas density measurements, recall that dRT/P = molar mass, assuming ideal gas behavior), which we DO know is 18.02 grams, then we can get the atomic masses of oxygen and hydrogen by multiplying the % mass of the element by the molar mass of the compound.
For example:

relative atomic mass of O = 88.90% x (18.02 amu per molecule) = 16.0 amu
relative atomic mass of H = 11.10% x (18.02 amu per molecule) = 2.00 amu

NOTICE that the atomic mass of H is incorrect! That is because water contains TWO H atoms per molecule.
Thus, the Cannizaro method is used for comparing the relative atomic mass of H in various compounds and THEN finding the SMALLEST DIFFERENCE in relative mass of H among the compounds. That difference most likely equals the atomic mass of H.

example:
in HCl,

H is 2.764% by mass and Cl is 97.236% by mass

multiply by the given molar mass ( from gas density measurements, recall that dRT/P = molar mass )of the compound:
relative atomic mass of Cl: 97.236% x 36.4609 amu per molecule = 35.453 amu
relative atomic mass of H: 2.764% x 36.4609 amu per molecule = 1.0078 amu

so the smallest difference in relative atomic mass (2.00 amu - 1.0078 amu) should be the actual atomic mass of H = .9922 amu which is pretty close to 1.0079 amu.

Naturally, to do gas density calculations for Zinc chloride, one would have to vaporize the salt AND assume ideal gas behavior...not a pretty combination. However, a mass spectrometer can be used to obtain the molar mass of any simple compound.