Transcript Lab Calculations Day

Mother’s Day is THIS
Sunday!
Wednesday, May 7th: “A” Day
Thursday, May 8th: “B” Day
Agenda
Finish lab: “Quantitative Determination of
an Empirical Formula” – if needed
Lab Calculations/Conclusion Questions
Start Chapter 7 Review
Extra Credit Opportunity!
Next time:
Finish Chapter 7 review
“Quantitative Determination of an
Empirical Formula”
Analysis
To find the mass of tin, Sn:
mass of dish, cover + tin
- mass of dish, cover
Remember
(should be ~ 2g)
sig figs!
To find mass of oxygen, O:
mass of dish, cover + tin (before heating)
- mass of dish, cover + product (after heating)
“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 of these by the smallest number of
moles.
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 for the tin-oxygen
product 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 and add %
“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.
 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.
 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.
 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 51 points, so make sure that all
of your data is labeled with units (grams, etc.)
and all of your calculations have the proper units.
Don’t forget your reflection statement!
Make sure your table of contents is updated.
Extra Credit Opportunity
Morrison High School
presents “The Wizard of
Oz”!
Friday, May 9th: 7:00 pm
Saturday, May 10th: 7:00 pm
Sunday, May 11th: 2:00 pm
Tickets: $5:00
Attend ANY performance
Have me sign your program
Bring program to class on
Mon/Tues
You will get 15
extra credit points!
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 due:
Thursday, May 15th: “A” Day
Friday, May 16th: “B” Day