Question Video: Identifying the Equation Used to Calculate the Concentration of Hydronium Ions | Nagwa Question Video: Identifying the Equation Used to Calculate the Concentration of Hydronium Ions | Nagwa

# Question Video: Identifying the Equation Used to Calculate the Concentration of Hydronium Ions Chemistry • Third Year of Secondary School

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Which of the following equations can be used to calculate the concentration of hydronium ions? [A] [OH⁻] = 10^(pOH) [B] [OH⁻] = 10^(−pOH) [C] [H₃O⁺] = 10^(−pOH) [D] [H₃O⁺] = 10^(−pH) [E] [H₃O⁺] = 10^(pH)

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### Video Transcript

Which of the following equations can be used to calculate the concentration of hydronium ions? (A) The concentration of OH− equals 10 to the pOH. (B) The concentration of OH− equals 10 to the negative pOH. (C) The concentration of H3O+ equals 10 to the negative pOH. (D) The concentration of H3O+ equals 10 to the negative pH. Or (E) the concentration of H3O+ equals 10 to the pH.

In order to determine which equation can be used to calculate the concentration of hydronium ions, we need to know what a hydronium ion is. Hydronium ions are formed in an acid–base reaction when water molecules accept a proton or H+ ion. The chemical symbol of a hydronium ion is H3O+. We can indicate the concentration of hydronium ions by placing the chemical symbol inside of brackets. We can see that answer choices (C), (D), and (E) contain the term “concentration of hydronium ions.” Answer choices (A) and (B) contain the term “concentration of hydroxide ions.” So we can eliminate these answer choices.

We can see from the answer choices that the concentration of hydronium ions must be related to either the pOH or the pH. Both the pH and pOH are unitless values that are used as a more practical method to represent the concentration of specific ions. The pOH can be calculated by taking the negative logarithm of the concentration of hydroxide ions, while the pH can be calculated by taking the negative logarithm of the concentration of hydronium ions. We can see then that the pOH is not directly related to the hydronium ion concentration. Therefore, we can eliminate answer choice (C).

We now know that pH and the concentration of hydronium ions are related. But we need to rearrange this equation in order to calculate the concentration of hydronium ions. To do this, we need a basic understanding of logarithms. The logarithm used in this equation is a base 10 logarithm. To better understand this function, let’s consider the value 1000, a multiple of 10. 1000 is equal to 10 times 10 times 10. In other words, 1000 is equal to 10 to the third power. The base 10 logarithm of a number is equal to the power to which 10 must be raised in order to obtain that number. So the base 10 logarithm of 1000 is equal to three.

Now that we have a basic understanding of logarithms, we can relate these two general equations to the pH equation. In the pH equation, the concentration of hydronium ions is 𝑥. But before we can determine 𝑦, we’ll need to move the negative sign. We can accomplish this by dividing both sides of the equation by negative one. This gives us negative pH equals the logarithm of the concentration of H3O+. Now we can see that negative pH is 𝑦 and the concentration of H3O+ is 𝑥. We can then write an equation following the format 𝑥 equals 10 to the power of 𝑦. This gives us the equation concentration of H3O+ equals 10 to the negative pH. This matches the equation shown in answer choice (D).

Therefore, the equation that can be used to calculate the concentration of hydronium ions is answer choice (D). The concentration of H3O+ equals 10 to the negative pH.

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