Question

(46-50.) Fish, like all animals, need a supply of oxygen, which they obtain from oxygen dissolved in the water. The minimum oxygen concentration needed to support most fish is around 5 ppm (w/v). How many moles of O2 per liter of water does this correspond to? (51-55). The Normal Saline solution used in medicine for nasal irrigation, wound cleaning and intravenous drips is a 0.91% (w/v) solution of sodium chloride in water. How would you prepare 1.5 L of this solution? (56-60). The maximum concentration of lead (Pb) which is considered safe in drinking water is 0.01 ppm. You receive a 250 mL sample of tap water which contains 0.05 of lead. Is this sample safe to drink?

Answer 1

To find the moles of O2 per liter of water for a concentration of 5 ppm, 5 mg of O2 is equivalent to 0.00015625 moles. To prepare 1.5 L of 0.91% Normal Saline solution, dissolve 13.65 grams of NaCl in sufficient water to total 1.5 L. A water sample containing 0.05 ppm lead in 250 mL exceeds the safe limit and is not safe to drink.

Step-by-step explanation:

To calculate the moles of O2 per liter of water for a minimum oxygen concentration of 5 ppm, we first recognize that 'ppm' stands for 'parts per million'. Since 1 liter of water weighs approximately 1000 grams, 5 ppm corresponds to 5 mg of oxygen per liter of water. Using the molecular weight of O2 (32.00 g/mol), we can convert this mass into moles.

5 mg O2 × (1 g / 1000 mg) × (1 mol / 32.00 g) = 0.00015625 moles of O2

Therefore, 5 ppm of oxygen is equivalent to 0.00015625 moles of O2 per liter of water.

To prepare 1.5 L of a 0.91% Normal Saline solution, we need to first calculate the mass of sodium chloride needed. 0.91% w/v means that there are 0.91 grams of solute per 100 mL of solution. To find the mass for 1.5 L:

(0.91 g/100 mL) × (1500 mL) = 13.65 grams of NaCl

Thus, to prepare 1.5 liters of the solution, dissolve 13.65 grams of sodium chloride in enough water to make the total volume up to 1.5 liters.

For the lead concentration in drinking water, a level of 0.01 ppm is considered safe. The given sample has a concentration of 0.05 ppm in 250 mL, which is above the safe limit. Therefore, this sample is not safe to drink.

Answer 2

To find the number of moles of O2 in water corresponding to 5 ppm, convert to grams and then to moles resulting in 1.5625 x 10⁻⁴ mol. To prepare 1.5 L of a 0.91% saline solution, dissolve 13.65 g of NaCl in less than 1.5 L of water and then adjust the volume to 1.5 L. The tap water sample with 0.05 ppm of lead is not safe to drink as it exceeds the maximum safe level of 0.01 ppm.

Step-by-step explanation:

To determine the number of moles of O2 per liter of water corresponding to 5 ppm, we can use the following steps:

• First, understand that 5 ppm means 5 mg of O2 per liter of water.
• Next, convert milligrams to grams: 5 mg is equivalent to 0.005 g.
• Since the molar mass of O2 is 32.00 g/mol, divide the mass in grams by the molar mass to find moles:
  0.005 g O2 ÷ 32.00 g/mol = 1.5625 x 10⁻⁴ mol of O2.

To prepare 1.5 L of a 0.91% (w/v) sodium chloride solution, follow these steps:

1. Calculate the mass of sodium chloride needed:
   0.91% of 1.5 L is 0.91 g/100 mL x 1500 mL = 13.65 g.
2. Measure out 13.65 g of sodium chloride.
3. Dissolve the sodium chloride in a volume of water that is less than 1.5 L.
4. Once dissolved, add more water until the total volume is 1.5 L.

Regarding the lead concentration in the tap water sample:

• The allowable concentration of lead is 0.01 ppm.
• The sample has 0.05 ppm, which is 5 times greater than the maximum safe level.
• Therefore, the sample is not safe to drink.