Question Description
Mixtures and Isotopes
What you will need:
homemade coathanger mass balance from Experiment 1 or storebought kitchen scale
three types of items similar in size that can be counted (a handful of each type); examples (pick one of these sets or make up your own):
desk supplies: thumb tacks, paper clips, and erasers
sewing supplies: buttons, safety pins, and spools of thread
coins: pennies, dimes, and quarters
candy: peanut M&Ms, regular M&Ms, and candy almonds
nuts: peanuts, almonds, and cashews
cereal: Cheerios, Lucky Charms, and Coco Puffs
mints: Lifesavers, Tic Tacs, and Altoids
Introduction
You will be using household items to create a mixture that you will then use to study two concepts: percent mass and average isotopic mass.
Percent mass is just what it sounds like Of the total mass of a mixture, what percent is the component you are interested in? Below is the general mathematical formula for percent mass.
Isotopes of an element have a common atomic number (number of protons) but differ in their numbers of neutrons. Average isotopic mass is the weighted average of the masses of the isotopes in an element. The calculation of average isotopic mass takes into account the percent abundance of each isotope and the mass of each isotope. Examples will be shown in the calculations section of this lab.
Notes for Submission
Print all pages on which you write answers to questions, perform calculations, or report data. Or, if you do not have a printer, write all work on blank pages of paper. Then take photos of your written pages, add in any photos of your experimental setup or materials, and turn these photos into a single pdf file. (Use an app like CamScanner, Adobe Scan, or Simple Scanner to do so.) Then upload the pdf into the assignment in Canvas.
Procedure:
Assemble the ingredients/items for your threecomponent mixture.
For steps 24 below, aim to get masses that are at least two significant figures. That is to say, avoid masses of 5 g, 10 g or 20 g. Shoot for masses like 11 g or 21 g.
Grab a handful of your first component (item type), and take its mass (in grams) using your homemade balance or your kitchen scale. Record the mass (total mass for item type) in a data table according to the example shown on the following page. Count the number of items in this handful, and report the number according to the table shown.
Grab a handful of your second component (item type), and take its mass (in grams) using your homemade balance or your kitchen scale. Record the mass (total mass for item type) in a data table according to the example shown on the following page. Count the number of items in this handful, and report the number according to the table shown.
Grab a handful of your third component (item type), and take its mass (in grams) using your homemade balance or your kitchen scale. Record the mass (total mass for item type) in a data table according to the example shown on the following page. Count the number of items in this handful, and report the number according to the table shown.
Take a fun photo of you showing off your kit/mixture! Submit this selfie with your lab writeup.
Sample Data and Calculations:
Sample Data Table for Desk Supplies Kit (Put your own numbers into the blank table provided.)
Item Type (component of mixture) 
Number or how many (exact values b/c counted)

Total mass for item type (g) (not exact b/c measured) 
% mass

% abundance 
Mass of one of this item type (e.g. one thumbtack) (g) 
thumbtacks 
28 
11 
19 
33 
0.39 
paper clips 
48 
19 
33 
56 
0.40 
endofpencil erasers 
10 
27 
47 
12 
2.7 
total 
86 
57 
99 (close enough!) 
101 (close enough!) 
N/A 
Average isotopic mass: __0.68____ g
Treating Desk Supplies Kit as a Mixture of Substances
(Model your calculations after these, and show them clearly.)
(Other two % masses calculated similarly. All reported in table above.)
Treating Desk Supplies Kit as Isotopes of an Element
(Model your calculations after these, and show them clearly.)
(Other two % abundances calculated similarly. All reported in table above.)
What if we wanted to describe the characteristics of a typical item in this kit? It would have to include traits of each of the item types but averaged in some meaningful way. Lets think about the average or representative mass of an item in this Desk Supplies kit. We would want to take into account that each type of item has a different mass. Plus, not all of the item types (components) are present in equal amounts. In other words, they have different percent
abundances. For example, the erasers have a large mass, but there are relatively few of them. So, the representative mass of the typical item is not close to the mass of the erasers, but it is much closer to the mass of the paper clips because there are many of them. Here is what we would do to get the overall (weighted) average:
Weighted average mass for item in kit with three item types (1, 2, 3)
= (% abund. 1)(mass 1) + (% abund. 2)(mass 2) + (% abund. 3)(mass 3)
If 1 = thumbtacks, 2 = paper clips, 3 = erasers,
Weighted average mass for item in kit
= (0.33)(0.39 g) + (0.56)(0.40 g) + (0.12)(2.7 g)
= 0.1287 g + 0.224 g + 0.324 g (keep extra digits for now, need two sig figs each)
= 0.6767 g (need to round to two decimal places)
= 0.68 g (average isotopic mass, rounded to two decimal places)
Is there an item in the kit that has this mass (0.68 g)? No. It is an overall average. Note that this average mass value is much closer to the mass of the paper clips and not close to the mass of the erasers. Why? Do the above calculations for your kit. This is the average isotopic mass that you need to report in your tables.
If we wanted to use the Desk Supplies kit to mimic a collection of isotopes of a particular element, we could pretend that there is an element called deskium (De) with three isotopes. One isotope is the thumbtack. We could call it deskium0.39. The second isotope would be the paper clips, which we could call deskium0.40. The third isotope, the erasers, would be deskium2.7. Note that the name of the isotope is the name of the element followed by a hyphen and its mass.
Make up some names for the components of your kit, assuming they represent isotopes of the same element. You could use candium (Cy), nuttium (Nu), etc. for the name of the element. Summarize your isotopes in a chart like the one below.
Sample Isotopes Data Table for Deskium (Make your own for your kit!)
Item Type (component of mixture) 
Name of this isotope 
% abundance 
Mass (g) 
thumbtacks 
deskium0.39 
33 
0.39 
paper clips 
deskium0.40 
56 
0.40 
endofpencil erasers 
deskium2.7 
12 
2.7 
Average isotopic mass: _____0.68______ g
For actual isotopes of elements, the units of grams for mass are too big. Atomic mass units are used instead (giving mass values that are greater than one). For example, there are two naturallyoccurring isotopes of boron. Because they are both boron atoms, they each contain five protons. (How do you know this? Where can you find this information?) Boron10 has a mass of 10.0129370 atomic mass units (amu). Its percent abundance is 19.9. That means 19.9 percent of the boron atoms in a typical sample are boron10 atoms. Boron11 has a mass of 11.0093055 amu and a percent abundance is 80.1. Shown below is the calculation of the average isotopic mass of boron.
Average isotopic mass of an element with two isotopes =
(% abundance in decimal form for isotope 1)(mass in amu of isotope 1)
+ (% abundance in decimal form for isotope 2)(mass in amu of isotope 2)
Average isotopic mass of B = (0.199)(10.0129370 amu) + (0.801)(11.0093055 amu)
= 1.992574463 amu + 8.818453706 amu (each 3 sig figs)
= 10.81 amu (rounded to two decimal places)
Does this final value match the average atomic mass of B on the periodic table??
Note that the final value is much closer to the mass of boron11 (11B) than to the mass of boron10 (10B). Why?
Your Data and Calculations: (Print and fill out or copy onto blank paper.)
Data Table for Kit
Item Type (component of mixture) (e.g. thumb tacks) 
Number or how many (exact values b/c counted)

Total mass for item type (g) (not exact b/c measured) 
% mass

% abundance 
Mass of one of this item type (e.g. one thumbtack) (g) 


















total 




N/A 
Average isotopic mass: _____________ g
Treating Kit as a Mixture of Substances
Space for calculations of percent mass for each item type (component):
Treating Kit as Isotopes of an Element
Space for calculations for percent abundance for each item type (component):
Space for calculations for mass of single item for each item type (component):
Space for calculation of weighted average mass (average isotopic mass) for your kit:
Isotopes Data Table for Kit
Item Type (component of mixture) 
Name of this isotope 
% abundance 
Mass (g) 












Average isotopic mass: _____________ g
PostLab Questions: (Print and fill out or write questions and answers on blank paper.)
You have a mixture that is 9.56 grams sugar, 8.22 grams of salt, and 0.556 grams of sand. What is the percent mass of the salt? Show your work clearly.
Nitrogen has two naturallyoccurring isotopes, nitrogen14 (14N) and nitrogen15 (15N). Based on the average isotopic mass of N from the periodic table, determine which of the two isotopes of N has the greater percent abundance. How do you know?
There are three naturallyoccurring isotopes of neon (Ne). The chart below summarizes the pertinent information about these three isotopes. Show the calculation for the average isotopic mass of neon.
Symbol of Isotope 
Name of Isotope 
% abundance 
Mass (amu) 
20Ne

neon20 
90.48

19.9924356

21Ne

neon21 
0.27

20.9938428

22Ne

neon22 
9.25

21.9913831
