Why does a neutral solution have a pH of 7.0?

Question

Accepted Answer

Welcome back, everyone. We need to explain why the P H of a neutral solution is seven. We have choices A through D which display different views on what constitutes a neutral solution. Either through the assertion of equal concentrations of hydro and hydroxide ions, their specific concentration values or the dominance of one ion over another. We're going to begin by recalling that P H is an indication of a solution's acidity or its basic recall that when we've got a P H of seven, that indicates a neutral aqueous solution. And so therefore, the concentration of hydro num H 30 plus is going to be equal to the concentration of hydroxide. And recall that we can calculate the concentrations of hydro num and hydroxide by using the water dissociation constant expression where we would recall that the K W of water is equal to the concentration of hydro multiplied by the concentration of hydroxide. And the K W of water is equal to a value of one times 10 to the negative 14th power. Since we stated that for a neutral solution, the concentration of hydro num should equal the concentration of hydroxide ions and the product of the two is equal to the K W of water, one times 10 to the negative 14th power. Then therefore, we can say that in a neutral solution where the concentration of hydro and hydroxide ions are equal, then the concentration of hydro and hydroxide individually would both equal the square roots of the K W of water. So in this case, the square root of one times 10 to the negative 14th power and that would result in the individual concentrations equal to one times 10 to the negative seventh power molar for each of our ions hydro and the concentration of hydroxide individually. With this outlined, let's recall our formula to calculate P where P can be found by taking the negative log of our concentration of hydro H plus. Because we know the individual concentration of hydro, we can verify ph by taking the negative log of the concentration of 1.7 times 10th, the negative 14th power. And that would result in the P H exactly equal to seven. Now to include as much precision as possible in the calculation of this neutral ph, we should include two digits after the decimal point being zeros. So two zeros after the decimal point. But looking at the prompt and answer choices, we are given the PH only to 10 after the decimal point. So the difference just lies in the number of significant figures where in our P H calculation of 7.0, we have three total sigfig but the prompt just simplifies 22 significant figures as 7.0 for the P H and both would be acceptable since the P H scale is a logarithmic scale and the difference between 7.0 and 7.0 is going to be significantly small and negligible in many contexts. So looking at our answer choices, we have choice a which states that a neutral solution has an equal hydro and hydroxide concentration of one times 10 to the negative seventh power molar resulting in a ph of 7.0 obtained from the negative logarithm base 10 of hydro. We would agree with what choice a outlines as a correct statement and correct explanation for why the ph of the neutral solution is seven. So let's consider that for now, now we have choice B which states that a neutral solution has an equal hydro and hydroxide concentration of 7.0 molar. We would automatically rule that out because that is incorrect and correct it to one times 10 to the negative seven power molar for each of the individual concentrations of hydro and hydroxide ions. So choice B B rule out because it does not correctly explain the neutral solutions P H of seven. Then we have choice C which states that a neutral solution has A P H of seven since the concentration of hydro is greater than hydroxide. And as we stated in a neutral solution, the concentration of our hydro num and hydroxide ions should be equal. And so we would rule out choice C for that reason, it is a false statement. Then we have choice D which states that a neutral solution has a P H of seven, since the concentration of hydroxide is greater than that of hydro, and we would rule out choice D for the same reason as choice C because the concentration of our ions should be equal. And so that leaves us with choice A as the only correct choice to correctly explain why the P H of a neutral solution is seven where a neutral solution has an equal concentration of hydro num and hydroxide ions of one times to the negative seven power molar resulting in A P H of seven obtained from the negative logarithm base 10 of hydro concentration. I hope that this made sense and let us know if you have any questions.

Why does a neutral solution have a pH of 7.0?

Key Concepts

pH Scale

Water Ionization

Equilibrium in Pure Water

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