Question

Phosphorus is present in seawater to the extent of \(0.07 \mathrm{Ppm}\) by mass. Assuming that the phesphorus is present as dihydrogenphosphate, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\), calculate the corresponding molar concentration of phosphate in seawater.

Short answer

The corresponding molar concentration of phosphate (H2PO4-) in seawater is approximately \( 2.26 \times 10^{-6} \) mol/L.

Step-by-step solution

Understand the given information

We are given the mass concentration of phosphorus in seawater as 0.07 ppm (parts per million). This means that there are 0.07 grams of phosphorus in every million grams of seawater. We are also given that the phosphorus is present as dihydrogenphosphate ions, H2PO4-. #Step 2: Convert ppm to grams per liter#

Convert ppm to grams per liter

We can convert ppm to grams per liter by considering that seawater has a density of approximately 1 g/mL (or 1000 g/L). Since there are 0.07 grams of phosphorus in every million grams of seawater, we can calculate the concentration in grams per liter as follows: Concentration (g/L) = (0.07 g P / 1,000,000 g seawater) × 1000 L seawater Concentration (g/L) = \( \frac{0.07}{1,000,000} \times 1000 \) = 0.00007 g/L #Step 3: Calculate the moles of phosphorus#

Calculate the moles of phosphorus

To find the molar concentration, we need to know the moles of phosphorus. We can use the molar mass of phosphorus (P) which is 30.97 g/mol to convert the grams of phosphorus to moles: Moles of P = \( \frac{0.00007 \mathrm{g P}}{30.97 \mathrm{g/mol}} \) #Step 4: Calculate the moles of dihydrogenphosphate ions#

Calculate the moles of dihydrogenphosphate ions

Since there is one phosphorus atom in each H2PO4- ion, the number of moles of dihydrogenphosphate ions is equal to the number of moles of phosphorus: Moles of H2PO4- = Moles of P #Step 5: Calculate the molar concentration#

Calculate the molar concentration

Finally, we can calculate the molar concentration of dihydrogenphosphate ions in seawater using the moles of H2PO4- and the volume of seawater. Since we calculated the mass concentration in grams per liter, we can consider the volume to be 1 liter: Molar concentration (mol/L) = Moles of H2PO4- / 1 L Molar concentration (mol/L) = \( \frac{0.00007}{30.97 \times 1} \) = \( 2.26 \times 10^{-6} \) mol/L The corresponding molar concentration of phosphate (H2PO4-) in seawater is approximately \( 2.26 \times 10^{-6} \) mol/L.

Key concepts

Parts per Million (ppm)

When analyzing substances like minerals in seawater, scientists often use the measurement of 'parts per million' - ppm. This unit is akin to a ratio that describes the number of units of mass of a contaminant per million units of total mass. To provide a clearer picture, imagine a small dash of spice mixed into a huge pot of soup. If we consider 1 ppm, it translates to 1 milligram of substance in 1 kilogram of water, or in volume terms, 1 milliliter of substance in 1 cubic meter of water.

Understanding ppm becomes particularly useful in environmental science for measuring levels of pollutants in water, air, or soil. It's an incredibly sensitive unit that permits the detection of even minuscule amounts of substances, which is crucial for health and safety regulations.

Conversion of Concentration Units

In scientific analyses, it's often necessary to convert concentration units to appropriately quantify and compare substances. For instance, converting ppm - which measures mass per mass - to molarity, which measures the number of moles per volume of solution, requires a few steps. Firstly, you need to take the density of the solution into account. Since seawater's density is approximately 1 g/mL, this simplifies conversion: 1 ppm equates to 1 mg/L directly due to the density of water.

The next step involves translating this mass into moles using the substance's molar mass. This conversion is vital for chemists and biologists to relate the mass concentration to the number of particles present, which influences reaction rates and biological effects.

Molar Mass

Molar mass is a fundamental concept in chemistry, representing the mass of one mole of a substance. It directly correlates the mass of a substance to the amount of substance (moles). In the periodic table, the molar mass is given for each element in grams per mole (g/mol). For compounds, the molar mass is the sum of the molar masses of its constituent elements.

For example, in our exercise with phosphate ions \( \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \), the molar mass of dihydrogenphosphate is calculated by summation of the molar masses of hydrogen (H), phosphorus (P), and oxygen (O) based on the number of each type of atom in the formula. Accurately knowing the molar mass is crucial for converting between grams and moles, which is an essential step in many stoichiometric calculations.

Stoichiometry

Stoichiometry is the area of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It is based on the conservation of mass and the concept of moles. By using balanced chemical equations, stoichiometry allows us to predict the amount of products formed from given reactants or the amount of reactants needed to create a certain amount of product.

In the context of our exercise, stoichiometry was used to relate the moles of phosphorus to the moles of dihydrogenphosphate ions. Since the stoichiometric coefficient for phosphorus in dihydrogenphosphate is 1:1, no additional conversions are needed, making for a straightforward stoichiometric relationship.