ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: gorillafunk on January 28, 2011, 07:05:25 pm
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Having problems with this question, help would be appreciated + karma :D
Lead exposure can cause permanent brain damage in infants and young children, even at very low levels. The amount of lead in infant milk formula can be measured by atomic absorption spectroscopy. A 2.5 g sample of milk powder was dissolved in 50 mL of distilled water. A very small volume of this solution was analysed and gave an absorbance of 0.130. Four standard solutions were analysed in the same way and the calibration graph in Figure 7.52 was obtained.
a Determine the concentration of lead (ng mL–1) in the diluted milk powder solution.
2.45 ng mL–1
b Calculate the concentration of lead (ng g–1) in the dried milk powder.
Wording confuses me.. tips would be awesome too!
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Is the answer 49 ng/g?
If so, What I did was basic multiplication of units like so...
1. We have a concentration in ng/mL (nanograms divided by millilitres)
2. We have 50ml of this solution
3. multiplying units: Concnetration (ng/mL) X Volume (mL) = ng/mL x mL = ng (units cancel out)
this is 2.45 X 50 = 122.5ng of lead in the 50ml sample
4. In the 50ml sample we have dissolved 2.5 grams of milk powder and want to find the amount of lead per gram (ng/g). We could say that there is 122.5ng of lead in 2.5 grams of milk powder...
However, dividing by 2.5 gives us the mass of lead in 1 gram of powder
122.5/2.5 = 49 ng/g
Note: When asked to find a concentration in the unit of x/y, simply determine the value for x and divide by the value of y. In this case, I found the number of nanograms of lead and divided by the number of grams of milk powder.
The ability to understand and manipulate units/ratios will make chemistry a lot easier for you this year!