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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: functions on October 05, 2011, 03:23:57 PM

Title: Can we determine the atomic weights of zinc or copper from these experiments?
Post by: functions on October 05, 2011, 03:23:57 PM

Experiment A

The mass of a copper turnings is determined. Heat the copper with excess sulfur. The product mass is determined.

Result: Mass of copper: 1.70g
           Mass of copper sulfide: 2.48g

Experiment B

The mass of zinc is determined. Add sufficient 6M HCl to the zinc to dissolve all the zinc. Heat the product on the steam bath until most of the liquid was disappeared. After that heat the product until the product looking somewhat pasty while hot. Weigh the product.

Result: Mass of zinc: 0.48g
           Mass of zinc chloride :1.02g

Can we determine the atomic weight of zinc or copper from this experiments? How? What additional information is necessary?

Title: Re: Can we determine the atomic weights of zinc or copper from these experiments?
Post by: Borek on October 05, 2011, 05:30:02 PM

You have to show your attempts at solving the question to receive help. This is a forum policy.

Title: Re: Can we determine the atomic weights of zinc or copper from these experiments?
Post by: functions on October 06, 2011, 03:10:20 AM

My way of thinking is as follow:

-Assume we do not konw the atomic weights of Zn or Cu, then we cannot convert the mass of Cu and Zn to the number of mole since if we know molar mass, we can directly convert it into atomic weight.

-comparing we the mass of Cu with the mass of the sulfur in copper sulfide cannot determine the atomic weight since we do not know the number of atoms of each element in the compound. We cannot determine the copper ion in the product is Cu²⁺ or Cu⁺ ion, the sulfur ion is S₂^- or S^2-.

-Zinc chloride is an ionic compound, Zn²⁺ combined with Cl^-. So, in a sample of ZnCl₂ the numbers of atoms of Cl is always two times the number of Zn. The mass of Cl in the product is 1.02g-0.48g=0.54g. Suppose there are x atoms of Zinc in ZnCl₂, then there are 2x atoms of Cl in ZnCl₂. Hence the mass of x atoms of Cl is 0.54g/2=0.27g. The ratio of mass of zinc to the mass of Cl: 0.48/0.27=16/9. Once we know the atomic weight of Cl, we can multiply it with 16/9 to get the atomic weight of Zn. Atomic weight of Cl is 35.45 amu, then atomic weight of Zn is 35.45 amu x 16/9= 63.022 amu.

Is there any other method can I use to determine the atomic weight of Cu?