Transcript calculate the empirical formula

Friday, May 10th: “A” Day
Monday, May 13th: “B” Day
Agenda
Finish lab: “Quantitative Determination of
an Empirical Formula”
Lab Calculations/Conclusion Questions
Start Chapter 7 Review
Next time:
Finish Chapter 7 review
“Quantitative Determination of an
Empirical Formula”
Analysis
Remember, 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”
Analysis
2. 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?)
“Quantitative Determination of an
Empirical Formula”
Conclusions
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”
Further Investigations
1. 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 total
molar 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 Investigations
Now, 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 Investigations
3. 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.
“Quantitative Determination of an
Empirical Formula”
Further Investigations
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
5. 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 review
Pg. 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