[Shipwreck]

A question in my review book is as follows:

2,92 grams of Cr₂O₃ is extracted from a 12,78 gram sample of Silver Chromate Ag2CrO4. What is the atomic weight of the Chromium?

The answer in the back of the book uses the periodic table molar mass values for oxygen and silver. But it seems to me that the question could produce various answers depending on which isotope of silver is found in the sample. In my mind it is impossible to determine the actual atomic weight of the chromium in question without first knowing the atomic weights of the other elements as well (oxygen is fairly straight forward since it is virtually always 16).

The atomic mass of silver provided is the average of several different isotopes. So if the isotope in question is, say, ¹⁰⁹Ag then the mass percentage of chromium in the sample would be slightly less than if the silver were, say, Ag-107. This would slightly affect the calculated value of chromium's atomic mass. It seems odd that the question is asking for a specific value for atomic weight but assumes average molar mass values. Wouldn't the resulting value be based on the average molar mass for silver?

[Arkcon]

When given a problem like this, we assume the average atomic mass of the element is what we're going to get for any handle-able quantity of the element.  No one warned you that the grams of compound were made of just one isotope, because it practice that isn't really possible to do, for a bulk quantity.

[Shipwreck]

Ok thanks.

The answer given is 51,75. The actual atomic mass for chromium is 51,996. I guess I just have difficulty wrapping my head around the idea that 2 out of the three elements in the sample have masses according to the periodic table, while the third varies significantly...somehow seems inconsistent to me...

[Dan]

The answer given is 51,75. The actual atomic mass for chromium is 51,996.

It is probably deliberately off to make sure the marker can tell whether you did the calculation, or just copied the atomic weight off a periodic table.

It is an odd question - use the periodic table to calculate a value given on the periodic table...

[Shipwreck]

It is an odd question - use the periodic table to calculate a value given on the periodic table...

  my thoughts exactly!