Question

3. Concentrations in solids and solution. You send a 10-g sample of rice to the lab for the determination of the total arsenic content. The analyst dissolves 1.0 g of the rice in 5.0 mL of concentrated acid and then dilutes the solution to a final volume of 25.0 mL. When analyzed by ICP-MS, the solution concentration is found to be 35 µg L^-1. What is the concentration of arsenic in the rice (in µg kg^-1)?   ___________ ppb

Hint: Follow the arsenic. You need to calculate the mass of arsenic in the rice, so you can divide that by the mass of the sample to calculate the concentration

4. Concentrations in solutions in mol per L (molarity M). If the molecular weight of a compound is 420 g per mol, how many mg would you weigh out to make 100 mL of a 0.0200 M solution? ________________ mg

Often, very concentrated solutions are specified by their mass percent composition. If 80% of the mass of a solution is a compound whose molecular weight is 48 g mol^-1, and the solution has a density of 1.20 g mL^-1. What is the molarity of the solution? ____________ M.

Hint: Take 1 L of solution, what does it weigh? So what is the weight of the dissolved compound? What’s the count in mol? This is the number of mol per L (the molarity, M).

5. Dilutions. If you take 10 mL of a 500 ppb solution and add sufficient water until the total volume is 250 mL and mix thoroughly, what is the concentration of the final solution in ppb? _________ ppb

At the food processing plant, 10 kg of rice containing 500 ppb of arsenic is thoroughly mixed with 240 kg of rice that is arsenic-free. What is the concentration of arsenic in the mixed rice in ppb?     ________________ ppb

Answer 1