Question 1

Why was the pH scale developed?

Accepted Answer

Chemist Soren Sorenson developed the pH scale in 1909 to provide a more convenient way to quickly compare the acidity of different solutions. Since hydronium ion concentrations can range from 1 M to 1 × 10−14 M, expressing these values directly is impractical. The pH scale uses logarithms to compress this enormous range into a simple 0-14 scale.

Question 2

How does a single pH unit change affect hydronium ion concentration?

Accepted Answer

Because pH is expressed on a logarithmic scale, a single-unit change corresponds to a 10-fold increase or decrease in hydronium ion concentration. For example, a solution with pH 3 contains ten times more hydronium ions than a solution with pH 4, and one hundred times more than a solution with pH 5. This logarithmic relationship makes pH a convenient way to express vast concentration differences.

Question 3

What is the relationship between pH and pOH in aqueous solutions?

Accepted Answer

The pH and pOH of any aqueous solution always sum to 14 at 25°C because the ion product of water (Kw) equals 1 × 10−14. Since pH is the negative logarithm of hydronium ion concentration and pOH is the negative logarithm of hydroxide ion concentration, this relationship allows you to calculate one value when the other is known. For instance, a solution with pOH of 10 has a pH of 4.

Question 4

How do acidic, basic, and neutral solutions differ in ion concentration?

Accepted Answer

An acidic solution has a higher concentration of hydronium ions than hydroxide ions and a pH less than 7. A basic solution has a lower concentration of hydronium ions than hydroxide ions and a pH greater than 7. A neutral solution has equal concentrations of hydronium and hydroxide ions with a pH of 7. These distinctions apply at 25°C, where Kw equals 1 × 10−14.

Question 5

What determines the pH range of 0 to 14?

Accepted Answer

The pH range of 0 to 14 is determined by the ion product of water (Kw), which equals 1 × 10−14 at 25°C. Taking the negative logarithm of this value yields 14, establishing the upper limit. The relationship pH + pOH = 14 ensures that aqueous solutions at standard temperature fall within this range, with pH values below 7 indicating acidity and above 7 indicating basicity.

Question 6

How does temperature affect pH classifications of solutions?

Accepted Answer

Temperature changes alter the ion product of water (Kw), which shifts the pH values that define acidic, neutral, and basic solutions. At 80°C, pure water has a hydronium ion concentration of 4.9 × 10−7 M, giving pH and pOH values of 6.31, making neutral solutions have pH = 6.31 rather than 7. This distinction is important when studying processes like enzyme reactions in warm-blooded organisms at 36-40°C.

Question 7

How is pH mathematically calculated from hydronium ion concentration?

Accepted Answer

pH is calculated as the negative logarithm of the hydronium ion concentration: pH = −log[H3O+]. For example, a solution with 1 × 10−5 M hydronium concentration has a pH of 5. This formula can be rearranged to find hydronium concentration from pH, allowing chemists to understand weak acid solutions and dissociation constant calculations when analyzing solution properties.

pH Scale

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