Friday, May 10 th : “A” Day Monday, May 13 th : “B” Day Agenda

Friday, May 10th: “A” DayMonday, May 13th: “B” Day

AgendaFinish lab: “Quantitative Determination of

an Empirical Formula”Lab Calculations/Conclusion QuestionsStart Chapter 7 Review

Next time:Finish Chapter 7 review

“Quantitative Determination of an Empirical Formula”

AnalysisRemember, to convert from grams to moles, use the

molar mass: To find moles of tin:

? g Sn X 1 mole Sn = 118.71 g Sn

To find moles of oxygen:

? g O X 1 mole O = 16.00 g O

Remember sig figs!

“Quantitative Determination of an Empirical Formula”

Analysis2. Determine the mole ratio of oxygen to tin in the

product.You know the moles of Sn and the moles of O.Divide each amount in moles by the smallest

number of moles you found.Did you get close to whole numbers?If not, multiply both numbers to get whole

numbers.These numbers are your mole ratio. (1:1, etc)

“Quantitative Determination of an Empirical Formula”

Conclusions

1. Using the mole ratio you just calculated, write the empirical formula and name it.

For example, if your mole ratio was 1 mole of Sn to 2 moles of O, your empirical formula would be SnO2.

Now, name the compound. (Hint: is this an ionic or covalent compound?)

2. What are the possible oxidation numbers for tin and oxygen? The 2 possible charges for the tin cation are

2+ and 4+. What is the charge on the oxygen anion?

Using these charges, write 2 possible empirical formulas for the tin-oxygen product. Does your empirical formula agree with either of these? If not, provide an explanation including any experimental errors that could have occurred.

“Quantitative Determination of an Empirical Formula”

Conclusions

“Quantitative Determination of an Empirical Formula”

Further Investigations1. Find the percentage composition of your

tin-oxygen product using the empirical formula you found.

Find the molar mass of your empirical formula Divide the mass contributed by

each element by the totalmolar mass

Multiply by 100 to change to %

“Quantitative Determination of an Empirical Formula”

Further Investigations

2. Analysis of a 20.30 g sample of a compound containing phosphorous and oxygen shows that 8.87 g of the sample are phosphorous. Calculate the empirical formula of the compound.

If 8.87 g of the sample is phosphorous, then the rest (11.43 g) is oxygen.

20.30 g – 8.87 g = 11.43 g

“Quantitative Determination of an Empirical Formula”

Further InvestigationsNow, use your notes to find the empirical

formula:Change grams of each to moles.Divide by the smallest number of moles you

found.If you didn’t get a whole number, multiply by

a small number to get a whole number.Write the empirical formula.

“Quantitative Determination of an Empirical Formula”

Further Investigations3. Using your notes, calculate the empirical formula

from the percent composition. Assume a 100 g sample so the percentages become

the same as grams. Change grams of each to moles using molar mass. Divide by the smallest number of moles you found. If you didn’t get a whole number, multiply by a

small number to get a whole number. Write the empirical formula.

4. The experimental molar mass from problem 3 was found to be 78 g/mol. Calculate the molecular formula.

Find the molar mass of the empirical formula from problem 3.

Put the bigger molar mass on top and divide to get your “n” value.

Multiply the empirical formula from problem 3 by the “n” value to get the molecular formula.

“Quantitative Determination of an Empirical Formula”

Further Investigations

“Quantitative Determination of an Empirical Formula”

Further Investigations5. Write the correct formula for the substance having

a mass of 322 g, and composed of 63.73% Na2S2O3 and 36.27% water.

Assume a 100 g sample so the percentages become the same as grams.

Change grams of each to moles using molar mass. Divide by the smallest number of moles you found. If you didn’t get a whole number, multiply by a

small number to get a whole number. Write the empirical formula.

“Quantitative Determination of an Empirical Formula”

Wow, that’s it! This lab is worth 46 points, so make sure that

all of your data is labeled with units (grams, etc.) and all of your calculations have the proper units.

Make sure your table of contents is updated.

Begin Chapter 7 Review(You’ll finish it in-class next time)

Chapter 7 reviewPg. 251-253: 2, 15, 19, 21, 23-24, 28-29, 35, 37, 42-

43, 45, 49-52, 55

Looking Ahead: Ch. 7 test/concept review/crossword

puzzle due: Thursday, May 16th: “A” Day Friday, May 17th: “B” Day