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.