Homework Help: AP Chemistry

Posted by T on Monday, May 18, 2009 at 11:37pm.

I need help with figuring out an appropriate way to do this sort of lab: how to determine an unknown compound.

With this one however, the teacher just gave us a white powdered compound - and asks us to figure out how to identify it using ANY lab procedures.

He gave hints that the powder DOES NOT react with water and it can be a SALT or just molecules.

I thought of doing a flame test, qualitative, finding molar mass, etc... but I don't know what to do specifically. Help?

Thank you!

AP Chemistry - DrBob222, Tuesday, May 19, 2009 at 12:00am
Is it just one compound? Is the compound an inorganic compound?
Color can give a hint.
A flame test is good if you know it's just one compound. Sometimes, but not always of course, it can identify the metal.
See if it's soluble in water. Then look at the table of solubilities. That will eliminate many compounds.
See it it reacts with dilute HNO3, HCl. dilute H2SO4? If it dissolves, does it do so with the evolution of a gas. If a gas, what's the odor of the gas? Again, consult with the solubility table for nitrates, chlorides, and sulfates. This should get you started.
[Note: For example, if it dissolves in water and a ppt forms with dilute HCl, you know immediately that it is one of three things. It must be a silver compound, a lead compound, or mercury(I) compound for those chlorides are insoluble in water. AgNO3, for exampl would be soluble in water but insoluble in dilute HCl.] I think what your prof wants you to do is to call on your knowledge of chemistry to create a reasonable procedure for systematic identification of ions.
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