2005 CSSA Trial : Question 12

[medhat tawfik]

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Hey everyone.

Oysters provide a rich source of zinc, an essential trace element in our diet. The minimum recommended intake for an adult is 12mg per day.

If a sample of oysters was found by AAS to contain an average of 200ppm of zinc per oyster how many oysters would an adult have to eat to reach the minimum recommended daily intake?

a) 0.2
b) 1.7
c) 6
d) 12

I know the answer, but don't know how to get to it. I would have thought you'd need to know the mass of each oyster. Help? Thanks in advance.

[Tangent]

Yeah, youd think that they would include the mass if they were dealing with ppm. I guess they want you more to look at the relationshipp between the numbers.

Stupid question

[mR sinister]

Sorry for the late reply :S

okk well the general concept when dealing with ppm is that
ppm = 1mg/1000g.

That is something you should know while reading up on AAS.

So by the given information..
The minimum daily intake = 12mg per day
= 0.012g per day
And an oyster contains 200ppm.
you know that ppm = 1mg/1000g ( 1000,000 mg = 1kg )
so 200ppm would equal
200 x 10^-6 g
= 0.0002 grams. of zinc present in 1 oyster.

you know the minimum daily intake is 0.012g

The answer should give you 60 oysters.
but that does not fit with any of the answers you have supplied.. either you made a typo,
or the question is wrong.

[medhat tawfik]

so 200ppm would equal
200 x 10^-6 g

That's what I'm not sure about. it's 200 x 10^-6 mg/g of oysters...but who says that 1 oyster weighs 1g?

As an example, if an oyster weighed 3 g, then by your calculations we'd only need 20 oysters. That's my issue. And no worries about late reply

[mR sinister]

I kind of don't get what you mean here,
Why would you need the mass of oysters?

it gives u the zinc content in 1 oyster, which is 200ppm
that is all you need to do this question. We don't look at the oyster as a whole, only the amount of zinc present in an oyster.

for example,
Lets say the minimum daily intake of iron for an adult is 2.5g
And if 1 oyster contains 0.0025g of iron

you would need to eat 1000 oysters, to gain the minimum amount of iron daily intake. Do you see how you would not need to know the mass of the oyster only its iron content

[medhat tawfik]

so 200ppm would equal
200 x 10^-6 g
= 0.0002 grams. of zinc present in 1 oyster.

That's where I disagree with you, because 200 PPM suggests that in every gram of oyster, there's 0.0002 grams of zinc. Basically, it gives you the zinc content as a fraction of the mass of the oyster (PPM), but it fails to give the mass of the oyster.

Example:

200 PPM = 0.0002 grams of zinc per gram of oyster. But the mass of an oyster is 10 grams.
200 PPM = 0.002 grams of zinc per 10 grams of oyster = 0.002 grams of zinc per oyster.

So to get 12 mg of zinc, you need 6 oysters. As the mass changes, the number of oysters needed changes...

Everyone I've asked has said the question is pretty much notorious for being stupid...so I guess it doesn't matter much anymore anyway. Didn't do well in chemistry trials so 1 multiple choice question won't kill me

[Akiheart]

Could you plz upload the paper and solns if u have them?

thx! =)