QUANTITATIVE DETERMINATION OF AN EMPIRICAL
FORMULA – TIN and OXYGEN
According to atomic theory, in ordinary chemical reactions an atom cannot be divided
into smaller parts. Therefore, when two elements combine they must do so in small whole
number ratios. For example, when hydrogen and oxygen react to form water, H2O, the
ratio of hydrogen atoms to oxygen atoms is 2 to 1, never 2.5 to 1 or 1.75 to 1.
In order to experimentally determine an empirical formula you will oxidize tin by treating
it with nitric acid (HNO3). You will then calculate the mole ratio of the two elements
(oxygen and tin) in one of the products and predict a formula for the resulting compound.
Procedure:
1. Clean and dry an evaporating dish and a watch glass cover. Be sure they are both
completely dry before you weigh them. Record their combined mass in the data
table.
2. Place a small amount of tin in the dish, cover with the watch glass, and measure
the mass. Record this in the data table.
3. Wearing goggles, add about 3 to 5 mL of nitric acid. Be sure to perform this step
in the fume hood. Nitric acid causes burns. Avoid skin and eye contact.
4. After the reaction has stopped, heat the dish (with the watch glass) on a hot plate
in the fume hood. Avoid spattering by reducing the heat. Continue to heat slowly
overnight, until the contents are completely dry.
5. When you come to lab the next day, reheat the dish on higher heat for a few
minutes and allow it to cool. Determine the mass of the dish, watch glass, and the
tin-oxygen product.
6. Heat the dish, watch glass, and product again on high heat for a few minutes.
Allow it to cool, then determine the mass of the dish, watch glass, and the tinoxygen product again. If the mass is within a few thousands of a gram as in step
5, you are done and you should record this mass in your data table. If it does not
agree with the measurement in step 5, repeat the heating, cooling, and weighing
process until the mass does not appreciably change.
Data Table:
Mass of evaporating dish and watch glass
_____________g
Mass of evaporating dish, watch glass, and tin
_____________g
Mass of evaporating dish, watch glass, and tin-oxygen product.
_____________g
Mass of tin
_____________g
Mass of oxygen
_____________g
Conclusions and Calculations:
1. From the data table above calculate the number of moles of tin. Be sure to show your
work
2. From the data table above calculate the number of moles of oxygen. Show your work.
3. Find the mole ratio of oxygen to tin in the product. Show your work.
4. Using the mole ratio you calculated from #3 above, predict the empirical formula for
the tin-oxygen product. Also, name this product.
Questions:
1. What are the possible oxidation numbers for tin and oxygen? Write the two most
probable empirical formulas for tin-oxygen compounds. Do your experimental results
agree with either of these? If not, provide an explanation.
2. From your data, determine the percent composition for each element in the tin-oxygen
product. Show work.
3. Analysis of a 20.30 gram sample of a compound containing phosphorous and oxygen
shows that 8.87 grams of the sample are phosphorous. Calculate the empirical formula of
the compound. Show work.
4. (a) Upon analysis a compound is found to contain 7.75% hydrogen and 92.25% carbon
by mass. Calculate the empirical formula for this compound. Show work.
(b) by other experiments, the molecular mass of the compound in (a) is found to be 78
grams per mole. What is the molecular formula? Show work.
5. Hydrates are compounds that contain water molecules. For example, when calcium
chloride crystallizes from a water solution, two moles of water are contained within one
mole of CaCl2. Hence, the formula for the hydrate is CaCl2.2H2O. The empirical formula
for a hydrate is determined by including the mass of water as part of the total mass of the
substance. Write the correct formula for the substance having a mass of 322 grams and
composed of 63.73% Na2S2O3 and 36.27% water. Show work.