Research and Calculations:
How many molecules of sulfuric acid (H2SO4) are in one liter of acid rain with a pH of 4?
In order to solve for this research question, I needed to do extensive research on the topic as well as expand my knowledge of various chemistry concepts. I first attempted to find the mass of the sulfuric acid in one liter of acid rain, yet after lots of research, I was unable to find any information on the amount of sulfuric acid. Instead, I decided to work backwards to find the amount of sulfuric acid from the acidity of the rain, leaving mass out of my calculations entirely. First, it is important to note that the acidity of a solution, or a solutions' pH, is measured by the hydrogen ion concentration in the solution. This can be found using the base-10 logarithm pH= -log[H+], where H+ is the concentration of hydrogen ions in moles per liter of the solution. Using this equation and the pH scale, I was able to determine that a solution with a pH of 4 has 1.0*10^-4 mol/L of H+, so 1 liter of acid rain with a pH of 4 has 1.0*10^-4 moles of H+. Using this information, I created the following conversion factor:
|
Next, I found out how much of the acidity of acid rain can be attributed to sulfuric acid. According to this website, about 75% of the acidity of rain is due to sulfuric acid, or in other words, about 75% of the H+ ions in acid rain come from sulfuric acid. So, out of 100 moles of H+, 75 moles of H+ are from sulfuric acid. Using this information, I created the following conversion factor:
|
Through research on sulfuric acid (H2SO4), I found that it is a strong acid, meaning that practically 100% of its molecules dissociate (break up into ions) in an aqueous solution. It is also a diprotic acid, meaning that it has the ability to donate two protons, or H+ ions, when dissociating. The dissociation of sulfuric acid in water is shown below in simplified form:
If we assume that both of these dissociation reactions occur every time, then it can be said that each sulfuric acid (H2SO4) molecule dissociates in water to give two hydrogen (H+) ions. If this happens, for every 1 mole of H2SO4, there are 2 moles of H+, as represented in the following conversion factor:
|
Finally, I had to convert from moles to molecules in order find the answer to my research question. To do this, I used Avogadro's number, 6.022*10^23, which is defined as the number of units (particles, molecules, atoms, ions, etc.) in one mole of any substance. In this case, there are 6.022*10^23 molecules of H2SO4 in 1 mole of H2SO4, as represented by the following conversion factor:
|
To find the final answer to my research question, the number of sulfuric acid (H2SO4) molecules in one liter of acid rain with a pH of 4, I multiplied all of my conversion factors together. All of the irrelevant units canceled out and left the final answer, as shown in the calculations below:
There are 2.25825*10^19 molecules of sulfuric acid in one liter of acid rain with a pH of 4.