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Ch.18 - Chemistry of the Environment
Chapter 18, Problem 38

Phosphorus is present in seawater to the extent of 0.07 ppm by mass. Assuming that the phosphorus is present as dihydrogenphosphate, H2PO4-, calculate the correspond-ing molar concentration of H2PO4- in seawater.

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1
Understand that 0.07 ppm (parts per million) means 0.07 grams of phosphorus per 1,000,000 grams of seawater.
Calculate the mass of phosphorus in 1 liter of seawater, assuming the density of seawater is approximately 1 g/mL, which means 1 liter of seawater weighs about 1000 grams.
Determine the mass of phosphorus in 1 liter of seawater using the ppm value: (0.07 grams of phosphorus / 1,000,000 grams of seawater) * 1000 grams of seawater.
Convert the mass of phosphorus to moles using the molar mass of phosphorus (approximately 30.97 g/mol).
Assume all phosphorus is present as dihydrogen phosphate (H2PO4-), and use the moles of phosphorus to find the molar concentration of H2PO4- in seawater.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Parts Per Million (ppm)

Parts per million (ppm) is a unit of measurement used to describe the concentration of a substance in a solution. It indicates how many parts of a substance are present in one million parts of the total solution. In this context, 0.07 ppm means that there are 0.07 grams of phosphorus per one million grams of seawater, which is essential for calculating molar concentration.
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Molar Concentration

Molar concentration, or molarity, is defined as the number of moles of solute per liter of solution. It is a crucial concept in chemistry for quantifying the concentration of a substance in a solution. To find the molar concentration of H2PO4- in seawater, one must convert the mass of phosphorus from ppm to moles and then relate it to the volume of seawater.
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Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). For dihydrogen phosphate (H2PO4-), the molar mass is calculated by summing the atomic masses of its constituent elements: hydrogen, phosphorus, and oxygen. This value is essential for converting grams of phosphorus to moles, which is necessary for determining the molar concentration in seawater.
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Related Practice
Textbook Question

An important reaction in the formation of photochemical smog is the photodissociation of NO : NO2 + hv → NO(g) + O(g) The maximum wavelength of light that can cause this reaction is 420 nm. (b) What is the maximum strength of a bond, in kJ/mol, that can be broken by absorption of a photon of 420-nm light?

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What is the molarity of Na+ in a solution of NaCl whose salinity is 5.6 if the solution has a density of 1.03 g>mL?

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Textbook Question

The enthalpy of evaporation of water is 40.67 kJ/mol. Sunlight striking Earth's surface supplies 168 W per square meter (1 W = 1 watt = 1 J/s). (a) Assuming that evaporation of water is due only to energy input from the Sun, calculate how many grams of water could be evaporated from a 1.00 square meter patch of ocean over a 12-h day

Textbook Question

The enthalpy of evaporation of water is 40.67 kJ/mol. Sunlight striking Earth's surface supplies 168 W per square meter (1 W = 1 watt = 1 J/s). (b) The specific heat capacity of liquid water is 4.184 J/g°C. If the initial surface temperature of a 1.00 square meter patch of ocean is 26 °C, what is its final temperature after being in sunlight for 12 h, assuming no phase changes and assuming that sunlight penetrates uniformly to depth of 10.0 cm?

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