Problem Set #12 Answers
1) One way to gauge the toxicity of a substance is to look up its LD50. A [larger][smaller] LD50 indicates that a substance
is more toxic.
2) Atoms, molecules, and formula units are so small that we count them by the mole. One mole is _6.02  1023___
particles.
3) The mass of one mole of a substance is called the _molar mass________________. You can find it
by __adding up all the atomic masses from the periodic table________________________________.
Problems:
4) The LD50 for botulinum toxin (commonly known as “botox”) is estimated to be about 2 ng/kg.
(a) Worldwide, the average weight of an adult is 137 lb. What is this weight in kg?
(b) Calculate the lethal dose of botox for an average adult, in ng.
(c) There are about 4.6 billion adults in the world. What mass of botox, in ng, would be required to deliver a lethal
dose to every adult on the planet?
(d) Given that 1 g = 1,000,000,000 ng, how many grams is your answer to part (c)? Does that seem like a lot? (For
reference, 1 paper clip weighs about a gram.)
This seems like a ridiculously small amount to kill everyone in the world.
5) Use the periodic table to determine the molar mass (in g/mol) of each of the following substances:
(a) Calcium = 40 g/mol
(b) K2CO3 = 2(39.1) + 12 + 3(16) = 138.2 g/mol
(c) Sulfur dioxide = SO2 = 32.1 + 2(16) = 64.1 g/mol
(d) N2O = 2(14) + 16 = 44 g/mol
(e) (NH4)2SO4 = 2[14 + 4(1)] + 32.1 + 4(16) = 132.1 g/mol
6) You buy a 500 mL bottle of water. The density of water is 1.0 g/mL.
(a) What is the mass of water in the bottle?
(b) How many moles of water are in the bottle?
(c) How many molecules of water are in the bottle?
7) You are given three samples of toxic substances:
(a) Determine the molar masses for gyromitrin and methanol and fill them in the table above.
gyromitrin = 4(12) + 8(1) + 4(14) + 1(16) = 100 g/mol
methanol = 1(12) + 4(1) + 1(16) = 32 g/mol
(b) Which sample contains the greatest number of molecules? Which sample contains the fewest molecules?
Explain your answer.
gyromitrin:
methanol:
strychnine:
The strychnine sample contains the fewest molecules (because it has the smallest number of moles).
The methanol sample contains the greatest number of molecules (because it has the greatest number of
moles).
(c) Which sample is the most toxic? Which sample is the least toxic? Explain your answer.
The way I would approach this is to imagine an animal with a mass of 1kg. For this animal, one lethal dose
would be equal to the number of milligrams in the LD50. How many lethal doses would be included in each
sample?
gyromitrin:
methanol:
strychnine:
67 lethal doses
5 lethal doses
1000 lethal doses
So the sample of strychnine is the most toxic, because it includes the most lethal doses. The sample of
methanol is the least toxic, because it includes the fewest lethal doses.
8) What would be the mass of 1 million molecules of sugar (C12H22O11)?
First, find the molar mass of sugar = 12(12) + 22(1) + 11(16) = 342 g/mol
To put that in perspective, our balances can measure down to a mass of 0.01 g, which is 10 -2 g. This is 108 times larger
than the mass of a trillion molecules of sugar. In other words, a trillion molecules is such a small number of molecules
that you would need 100 million times that many molecules just to weigh them on one of our balances.
9) Suppose you run a company that buys copper compounds and then recycles the copper for resale. Your company
wants to get the most pure copper for the lowest cost. Three different suppliers want to sell you 1 mol CuO(s), 1 mol
CuCO3(s), and 1 mol Cu2O(s) for the same price.
(a) Which compound has the greatest total mass? Show your work.
CuO = 63.55 + 16 = 89.55 g/mol
CuCO3 = 63.55 + 12 + 3(16) = 123.55 g/mol
Cu2O = 2(63.55) + 16 = 143.1 g/mol
Since we have one mole of each, the Cu2O sample is the heaviest (it has the greatest molar mass).
(b) Which compound has the greatest mass of Cu? Show your work.
In one mole of CuO, there is one mole of Cu. So this sample contains 63.55 g of Cu.
In one mole of CuCO3, there is one mole of Cu. So this sample contains 63.55 g of Cu.
In one mole of Cu2O, there are two moles of Cu. So this sample contains 127.1 g of Cu.
(c) Assuming it costs the same to extract the copper from each compound, which compound represents the best
deal for your company? Explain.
Since they’re all the same price, the one that contains the most copper is the best deal. That means the Cu2O
sample.