Question

In order to analyze water samples using a spectrophotometer or plate reader, it is necessary to turn the molecules of nitrate into a dye molecule that can be quantified. The first step in turning nitrate (NO₃^-)  into a dye molecule is reducing it to a molecule of nitrite (NO₂^-). This is done by reacting the NO₃^- with cadmium.

After the reduction reaction, the NO₂^- is reacted with two additional reagents. The first reagent, Reagent A, is a solution of sulfanilamide and hydrochloric acid. The second reagent, Reagent B, is a solution of N-(1-naphthyl)-ethylenediamine, called NNED for short. The compounds are mixed with the water sample and produce a purple color. The intensity of the purple color is directly related to the concentration of nitrite in the water sample. We can measure how purple the water turns as absorbance on a spectrophotometer and then convert the absorbance to concentration of nitrate.

To make Reagent A, we will need to make a solution of 10.0 g of sulfanilamide in 1 L of 2.4 molar hydrochloric acid (HCl).

The stock solution of HCl is 12 molar HCl. How many milliliters (mL) of 12 M HCl would you add to produce 0.3 liters (L) of 2.4M HCl? ____________ mL HCl

After creating 0.3 L of 2.4 molar HCl solution, how many grams of sulfanilamide will be added? ____________ g sulfanilamide

Part 2

After reacting the nitrate with cadmium to produce nitrite, the nitrite is then reacting with sulfanilamide and N-(1-naphthyl)-ethylenediamine, to produce a purple dye molecule that can be quantified on a spectrophotometer.

The N-(1-naphthyl)-ethylenediamine, called NNED for convenience, reagent is made by mixing 1 gram of NNED in 1 liter of water. However, we don't always want to make an entire liter of solution because the NNED solution only lasts about 1 month before going bad and turning brown.

How many milligrams of NNED will need to be added to make 0.2 liters of solution? __________

Answer 1

Ans. 12 molar HCl implies that 12 moles of the solute are present in 1000ml of the solution. Therefore, to produce 1 litre (1000ml) of 2.4 M HCl, we will need to add 2.4/12*1000 = 200ml of the 12 M HCl.
     Therefore, to produce 0.3 litre of 2.4 M HCl, we will need to add 0.3*200 = 60ml of 12 M HCl.

To produce reagent A, 10.0g of sulfanilamide needs to be added to 1 litre of 2.4 M HCl. Therefore, 10*0.3 = 3g of sulfanilamide needs to be added to 0.3 litre of 2.4 M HCl.

Part 2:

To produce 1 litre of solution, 1g of NNED is added with 1 litre of water. Therefore, to produce 0.2 litre of solution, 0.2*1g = 0.2g (=0.2*1000mg) = 200mg of NNED needs to be added.