1. What is the mass, in miligrams, of mercury in a 1-kg...

1. What is the mass, in miligrams, of mercury in a 1-kg lake trout which just meets the North American standard of 0.5 ppm Hg? What mass of fish, each at the 0.5-ppm Hg level, would you have to eat in order to ingest a total of 45 mg of mercury? 2. The U.S. EPA recommends a maximum dietary intake for methylmercury of 0.1 µg per day per kilogram of the human's body weight. What mass of fish can a 55-kg woman safely eat each week if the average methylmercury level in the fish is 0.30 ppm? 3. According to an informal 1992 survey, the drinking water in about one-third of the homes in Chicago had lead levels about 7 ppb. Assuming that an adult drinks about 2 L of water a day, calculate the total lead that residents of these Chicago homes obtain daily from their drinking water.

Transcript

00:01 Find the mass in milligrams of mercury in a one kilogram fish given that there is 0 .5 parts per million of mercury in the fish.

00:10 Then we have to find the amount of fish or the mass of the fish needed to get to 45 milligrams of mercury.

00:17 So the first thing we're going to note is the unit ppm.

00:20 Now if you're familiar with this in a solution form you might recognize it as grams per liter.

00:27 Since we're working with mercury it's going to be grams of mercury per liter and this liter is water.

00:33 But since we're working in a solid and not a solution what this is going to turn into is grams of mercury per one kilogram of fish.

00:45 So 0 .5 parts per million is really 0 .5 grams of mercury per one kilogram of fish.

00:54 So that answers question part a the first part.

00:58 So we have to convert this to milligrams.

01:00 So we're going to say one gram on the bottom for every 1000 milligrams that's going to be on the top of our fraction.

01:07 So we have 0 .5 multiplied by 1000 and that's going to be 500 milligrams for every one kilogram of fish.

01:17 So now we have to find the mass of fish needed to reach 45 milligrams of mercury.

01:24 So we can just use 45 milligrams of mercury and we can use the ratio of 500 milligrams to one kilogram of fish.

01:33 So we're going to flip that over say one kilogram of fish for every 500 milligrams of mercury.

01:41 So we take 45 we divide it by 500 and this will give us 0 .09 kilograms.

01:49 Let's put this in grams.

01:51 So here we'll say one kilogram for every 1000 grams.

01:55 So let's take our previous answer multiply it by a thousand and this will yield 90 grams of fish.

02:03 So now let's take a look at question two.

02:06 The limit for methyl mercury we were given as 0 .1 micrograms of methyl mercury for one kilogram of body weight.

02:12 Now this is body weight for a person.

02:16 Find the mass of fish a 55 kilogram person would need to eat if the concentration of mercury in the fish is 0 .30 parts per million.

02:24 So what we're going to do is we are going to start with 55 kilograms of body weight.

02:30 We're going to use the limit of methyl mercury.

02:33 So we're going to say for every one kilogram of body weight the limit is 0 .1 micrograms of our methyl mercury.

02:42 I'm just going to shorten that to mercury.

02:44 So now we're in micrograms of mercury.

02:47 Now remember this 30 parts or 0 .30 parts per million is really 0 .30 grams of mercury for one kilogram of fish.

02:59 So let's convert these micrograms of mercury to grams of mercury...

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