Relative Mass Packet
Relative.....relative.....relative. What does relative mean? We have relatives, but many times things are said to "be
relative", and chemistry reports the relative mass for all of the chemical elements. So what is the meaning of the
term relative?
Simply put, relative means "in comparison". It's a RATIO!! Relatives are people who come from the same family
as you, so in a sense you are relative to them. In chemistry we keep track of types of atoms by using their relative
mass, their mass in comparison to other atoms. For example, oxygen has a relative mass of 15.9994 and hydrogen
has a relative mass of 1.008, so oxygen atoms are roughly 16 times more massive than hydrogen atoms.
Activity One: Examining Relative Mass
The purpose of activity one is to give insight into what is meant by ‘relative’ mass, which is extended to
conceptualize what is meant by molar mass.
1. You have an assembly of two plastic tubes on opposite ends of a ruler.
2. Lift the ruler and tube assembly by picking up the middle string. The ruler should be balanced. If the ruler is
not balanced, add small pieces of wire to the side of the ruler that is tilted upward until the ruler is level or as
close as possible.
3. Begin by adding 20 BB’s to the tube on the left. Rest the assembly on the desktop while doing this part of the
activity. Now add the smaller GB’s (glass beads) to the other tube until you think the ruler will balance.
Carefully lift the assembly from time to time to determine if the ruler is balanced.
4. Once you think you have achieved a balanced condition, hold the balanced assembly by the middle string in
front of you. Try to line the top edge of the ruler with a level surface such as a window sill or some piece of
furniture to determine if the ruler is level. The process is imperfect, but try to get it as level as possible.
5. When the ruler is satisfactorily balanced, lower the balance to your desktop, and remove the GB’s from the tube
and count them.
6. Using the information gained from counting the GB’s, predict how many of these would be required to balance
10 of the BB’s or pellets. My prediction is ______________.
7. Repeat the experiment using only 10 BB’s and determine how many GB’s are required to make the ruler
balance.
Findings:
1. In your experiment how many times more massive was the BB relative to the GB? **Notice that this is a relative
mass. We do not know the exact mass of either the large or small pellet.
2. If a GB had been assigned a mass of 12 relative mass units (rmu), then what is the mass of the BB in rmu?
**Notice that 12 relative mass units is a term that does not measure mass in grams. Think of it as a relative term as
opposed to a measured term.
3. If a single GB has an actual mass of 0.012 grams, how many grams would 1000 GB's have?
4. Suppose that a passel is a thousand of anything (glass beads, BB’s or atoms) then what is the passel mass (mass
of a passel) of the GB’s if a single GB has a mass of 0.023 grams?
5. Particle to particle, the BB is how many (write in your data)_____ times as massive as the GB? Therefore, the
passel mass of the BB is ________ grams.
***We know we cannot truly mass a single atom. We need lots of atoms to determine mass with our lab balances. Suppose we
say 12.0 grams of carbon is the number of atoms in some unit called the mole. At this point we really don’t need to know
what the value of the number is.
6. If a titanium atom has a mass that is 3.99 (4.0 for practical purposes) times as massive as a carbon atom:
A) How many carbon atoms would come very close to balancing one titanium atom?
B) Suppose that we have 12.0 grams of carbon atoms and 12.0 grams of titanium atoms. There would be (how
many)_______ times as many carbon atoms in 12.0 g as there are titanium atoms in 12.0 grams.
C) How many moles of titanium atoms are present in 12.0 grams of titanium, if 12.0 grams of carbon is
defined as 1.0 mole of carbon?
D) Suppose that it is learned that you have 1.505 x 10^23 titanium atoms in 12.0 grams of titanium atoms.
How many carbon atoms are present in 12.0 grams or 1.0 moles of carbon?
Activity Two: Determining Relative Mass; An Analogy
How were the relative masses of the atoms originally determined? By experiment of course!! Lets use an analogy
(a comparison of things typically thought to be not alike, e.g. life is like a river) to learn more about relative mass.
Coins, like atoms, have mass and value. A dime contains a specific amount of matter and has the value of 1/10 of
a dollar. Like a dime, a nitrogen atom contains a specific amount of matter and combines with other elements in a
manner specific to nitrogen.
Using John Dalton's Atomic Theory, which states that atoms combine in simple whole number ratios, and the
concept of relative mass, chemists determined the relative masses of atoms by analyzing chemical compounds.
Specifically, by measuring the resulting mass, or mass of products, of a chemical reaction and comparing it with
the relative masses of reactants. Lets use collections of coins to illustrate this process.
Listed below are data for simple combinations of coins (pennies, nickels, etc.). A relative mass and a total value
are given for each combination. Since relative mass is a comparison, the numerical value is unit-less, the units
cancel (10 g/5g = 2). For the sake of keeping track of numbers and measurements we will define the units of
relative mass as the "relative mass unit" or rmu.
Can you figure out the term we use for the relative mass of atoms? (how about "atomic mass units"!!)
2 rmu = 5 cents
4 rmu = 7 cents
8.2 rmu = 37 cents
4.4 rmu = 50 cents
2.9 rmu = 12 cents
Based upon the information above see if you can deduce the relative masses for the following coins.
Value
penny
nickel
dime
quarter
Relative Mass
Actual Mass
1. When you think you have the relative mass of each coin, check your work and answers with your teacher.
2. Once relative masses are known, determining one "actual" mass means that all of the others can be computed.
Obtain a coin from your teacher and use a balance to determine it's actual mass. Using the coin's actual mass
and the relative masses of the other coins, determine each coin's actual mass and record their values.
Assuming that coins, like atoms, combine in simple whole number ratios, the total mass of a collection of coins
must be equal to the sum of the masses for all of the coins in the collection. For example, the total mass of a
collection of two pennies and five nickels must be 2(2.54g)+5(5.00g)=30.08g. We could also use the short hand
formula P N to symbolize this compound.
2 5
3. Obtain a vial containing an example "penny-nickel" compound. By measuring the mass and using your
powerful mind, determine the ratio of pennies to nickels in the vial. Report the ratio as a formula for the
compound (Don't forget to account for the mass of the vial).
Open the vial and check your calculations.
If your calculations were incorrect obtain another vial and try again.
Aren't mass ratios a powerful way to count and keep track of things you cannot see!!
We can use the same thinking process with atoms that we have used with coins. The atomic theory tells us that
each atom of the same element will have the same mass. By analyzing compounds, the relative mass of the
elements can be determined. The relative mass of an element is it's mass compared to a standard atom. Dalton
chose hydrogen as his standard since it was the lightest element. Today we use an atom of carbon. Lets look at a
chemical reaction where copper and oxygen react to form a copper-oxygen compound.
copper
+
oxygen
=
copper oxide
Starting with 16.00 g of oxygen, 80.12 g of copper oxide is produced.
4. What mass of copper reacted with the oxygen?
5. If the relative mass of oxygen is known to be 16.00 amu, then this reaction uses one unit of oxygen. If copper
and oxygen react on a 1:1 unit basis, what is the relative mass of copper?
6. If the copper and oxygen happened to react on a 1:2 unit basis, then the entire mass of copper that reacted
represents two units of copper instead of one. If the ratio is actually 2 coppers to 1 oxygen, then what is the
relative mass of copper?
7. From the experimental data it is impossible to determine the reaction ratio for units of oxygen to units of
copper, let alone the relative mass of copper. To determine the relative mass of copper we need another
chemical reaction that uses copper as a reactant. This process is similar to the thinking pattern you used in
determining the relative masses for the coins. Think about how this is possible and explain it to your teacher.
Activity Three: Determining relative atomic mass by experimentation
In this experiment you will synthesize a compound, zinc chloride, and determine the mass ratio of zinc to
chlorine. Using the experimental data, the relative mass for each element and the chemical formula of zinc
chloride will be determined. The reaction in words:
zinc
+
hydrochloric acid
----->
zinc chloride
+
hydrogen gas
Construct a data table which should include:
First Day:
Second Day:
mass of beaker
mass of beaker + zinc
mass of zinc
mass of beaker + solid (zinc chloride)
mass of zinc chloride
mass of chlorine that combined with the zinc
A. First Day:
1. Weigh a clean, dry 100 ml beaker and label it with your lab group’s name.
2. Add 0.65 g of granular zinc to the beaker and record the exact mass.
3. Add 15 mL of dilute hydrochloric acid and record your observations.
4. Set the beaker aside to react completely overnight.
B. Second Day:
1. Evaporate the solution to dryness over a burner. Heat gently, as strong heating may result in splattering
your product.
2. Extended heating will melt the solid. When the solid melts stop heating.
3. When the beaker has cooled, weigh the beaker and solid.
4. Wash the beaker with water to remove the solid.
Findings
1. Calculate the mass ratio of chlorine to zinc in your sample. Compile your ratio with your classmates and
compute a class average ratio for chlorine to zinc.
2. You started with 1/100 the relative mass of zinc, 0.65g. If each atom of zinc combined with one atom of
chlorine, the formula of zinc chloride would be ZnCl. The mass of zinc chloride would be
zinc
chlorine
zinc chloride
0.65 g
0.35 g +
1.00 g
If each atom of zinc combined with two atoms of chlorine, the formula of zinc chloride would be ZnCl 2. How
much zinc chloride would be formed?
3 From separate experiments it is determined that oxygen has a relative mass of 16.00 and in the compound ZnO
the zinc to oxygen mass ratio was 4.086 to 1. What is the relative mass of zinc?
4. Assume that you had one zinc atom for each chlorine atom in your sample. Using the class average ratio,
calculate the relative mass of chlorine.
5. If you had two chlorine atoms for each zinc atom in your sample what would be the relative mass of chlorine?
6. In another experiment a chlorine to oxygen ratio of 2.219 to 1.00 was found (O = 16.000 amu). Calculate the
relative mass of chlorine in this experiment. Based upon the data from this experiment and your experiment,
does zinc chloride have one atom of zinc to one atom of chlorine or does it have one atom of zinc to two atoms
of chlorine? Write a valid formula for the compound zinc chloride (ex. CuCl ) and explain your selection.
2
Further Extension: The mole concept
1. A mole is the SI unit of amount. One mole is equivalent to 6.02 x 10^23 particles. The mass of 6.02 x 10^23
particles varies with the type of particle. One mole or 6.02 x 10^23atoms of carbon has a mass of 12.0 grams.
Since titanium atoms are 3.99 times as massive as carbon atoms, then what is the mass of one mole of titanium
atoms?
2. The mass of one mole is also called the molar mass. If lithium atoms are 0.578 as massive as carbon atoms,
what is the molar mass of lithium if the molar mass of carbon is 12.0 grams?
3. If 6.02 x 10^23 atoms of Mg are 2.025 times as massive as carbon atoms what is the mass of one mole if carbon
having a molar mass of 12.0 grams?
Activity Four: Bringing things together
Use the following terms to write a summary paragraph. Underline the words as you use them. You may use
your textbook as a reference.
comparison
amu
6.02 x 10^23
particles
relative
carbon
standard
atomic mass unit
molar mass
ratio
mole
Avogadro's number