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Chem practice Set 1 Question 17- LD50
Emily_6689
#1 Posted : Sunday, June 13, 2021 10:42:37 AM
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Hi, the answer explanation for the correct answer for #17 seemed to come completely out of left field? is this formula for LD50 something we should know? (if so it is not in the equation sheet in our CC)


I can't find anywhere in the passage that would suggest that a 'lighter option' would be more harmful.

Similarly, how do we know to assume that "all of the toxicity of NaCl comes from Na+".... and the equation for finding LD50 of just Na+... etc


Thanks in advance.
INSTR_Katrina_128
#2 Posted : Monday, June 14, 2021 2:16:56 PM
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Hi Emily!

Great question.

LD50 is something that you might encounter in the future, and is often used to quantify toxicity and "risk" associated with a chemical on Material Safety Data Sheets (MSDS). LD50 stands for "lethal dose 50%" -- it is the lethal dose that will kill 50% of a population. How is it found? Well... we typically test LD50 with a group of test animals. Then, the data is extrapolated to apply to humans, because animals and humans have different physiologies. What would kill a tiny animal probably won't kill a human.

OK, so now let's answer the question.

Notice the UNITS on LD50: mg/kg.

This represents the mg of chemical per kg of body mass. How many mg of this chemical per kg of human body mass will it take to kill half of a sample of humans? Great. 😐

Answers C and D are ions that are PART of two of the compounds in the table. Let's analyze NaCl and Na+:

Based on the periodic table, we can figure out the molar mass of NaCl and Na+:

NaCl = 23 + 35 = 58 g/mol

Na+: 23 g/mol

Then, taking a ratio, we can see that Na+ contributes 23/58 = 1/3 (approx) of the total mass to the NaCl molecule. Therefore, we can approximate that Na+ contributes 1/3 of the LD50 to NaCl, or approximately 1000 mg/kg.

OK. Let's do the same for NaCN and CN-.

NaCN = 23 + 12 + 14 = 49 g/mol

CN- = 12 + 14 = 26 g/mol

Then, taking a ratio, we can see that CN- contributes 26/49 = 1/2 (approx) of the total mass to the NaCN molecule. Therefore, we can approximate that CN- contributes 1/2 of the LD50 to NaCN, or approximately 3.2 mg/kg.

Since this is the lowest value of all the possible answers, D is the best answer.
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