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2Na(s)+Cl 2 (g) 2NaCl(s)

A chemist determines that the mass of an empty flask with a stopper is 156.1870g. After filling the stoppered flask with chlorine gas (Cl2 ) it’s mass is 158.1743. The chemist introduces 0.3870g of elemental sodium (Na) into the flask, causing a violent reaction and creating NaCl. All of the solid sodium is used up by the reaction. The chemist collects the unreacted chlorine gas and determines its mass to be 0.7962 g.

1. How many moles of elemental sodium were used in the reaction?

2. What is the mass of chlorine gas used in the reaction?

3. How many moles of chlorine were used in the reaction?

4. What is the empirical formula of sodium chloride based on the experimental data?

5. Was the empirical formula you obtained correct using the chemists data correct? Why?

Explanation:

1. How many moles of elemental sodium were used in the reaction?

Since all of the solid sodium is used up by the reaction, you can cancluate the number of moles of elemental sodium used dividing the mass by the molar mass:

2. What is the mass of chlorine gas used in the reaction?

a) Mass of chlorine gas introduced in the flask = mass of the stoppered flask after filling it with chlorine gas - mass of the empty flask with the sopper

b) Mass of chlorine gas unreacted = 0.7962 g (given)

c) Mass of chlorine gas used = mass of chorine gas introduced in the flask - mass of chlorine gass un reacted

3. How many moles of chlorine were used in the reaction?

4. What is the empirical formula of sodium chloride based on the experimental data?

Divide the number of moles of each element by the smalles number of moles:

Multiply by 2 to obtain whole numbers:

5. Was the empirical formula you obtained correct using the chemists data correct? Why?

No, the empirical formula you obtained using the chemists data is not correct, because the correct empirical formula of sodium chloride is NaCl.

That is, there is 1 atom of sodium per every atom of chlorine in one chemical formula of NaCl, but that is not reflected by the empirical formula Na₂Cl₃.

That is a demostration of big experimental errors. You can speculate that the errors are likely due to problems of procedure collecting the gas or errors in measuring the masses.

As you see, the masses are measured to thousandths of grams, which requires much precision; thus smalls absolute errors could produce huge relative errors.

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