COFFEE HAS A pH 5. IS IT ACIDC, BASICOR NEUTRAL?
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Answers
Answer:
"Each number represents a 10-fold change in the acidity/basicness of the water. Water with a pH of five is ten times more acidic than water having a pH of six. As this diagram shows, pH ranges from 0 to 14, with 7 being neutral. pHs less than 7 are acidic while pHs greater than 7 are alkaline (basic)."
Explanation:
2.6 The pH Scale
LEARNING OBJECTIVES
Define pH.
Determine the pH of acidic and basic solutions.
As we have seen, [H+] and [OH−] values can be markedly different from one aqueous solution to another. So chemists defined a new scale that succinctly indicates the concentrations of either of these two ions.
pH is a logarithmic function of [H+]:
pH = −log[H+]
pH is usually (but not always) between 0 and 14. Knowing the dependence of pH on [H+], we can summarize as follows:
If pH < 7, then the solution is acidic.
If pH = 7, then the solution is neutral.
If pH > 7, then the solution is basic.
This is known as the pH scale. You can use pH to make a quick determination whether a given aqueous solution is acidic, basic, or neutral.
EXAMPLE 12
Label each solution as acidic, basic, or neutral based only on the stated pH.
milk of magnesia, pH = 10.5
pure water, pH = 7
wine, pH = 3.0
Solution
With a pH greater than 7, milk of magnesia is basic. (Milk of magnesia is largely Mg(OH)2.)
Pure water, with a pH of 7, is neutral.
With a pH of less than 7, wine is acidic.
Test Yourself
Identify each substance as acidic, basic, or neutral based only on the stated pH.
human blood, pH = 7.4
household ammonia, pH = 11.0
cherries, pH = 3.6
Answers
basic
basic
acidic
Table 12.3 "Typical pH Values of Various Substances*" gives the typical pH values of some common substances. Note that several food items are on the list, and most of them are acidic.
Table 12.3 Typical pH Values of Various Substances*
Substance pH
stomach acid 1.7
lemon juice 2.2
vinegar 2.9
soda 3.0
wine 3.5
coffee, black 5.0
milk 6.9
pure water 7.0
blood 7.4
seawater 8.5
milk of magnesia 10.5
ammonia solution 12.5
1.0 M NaOH 14.0
*Actual values may vary depending on conditions.
pH is a logarithmic scale. A solution that has a pH of 1.0 has 10 times the [H+] as a solution with a pH of 2.0, which in turn has 10 times the [H+] as a solution with a pH of 3.0 and so forth.
Using the definition of pH, it is also possible to calculate [H+] (and [OH−]) from pH and vice versa. The general formula for determining [H+] from pH is as follows:
[H+] = 10−pH
You need to determine how to evaluate the above expression on your calculator. Ask your instructor if you have any questions. The other issue that concerns us here is significant figures. Because the number(s) before the decimal point in a logarithm relate to the power on 10, the number of digits after the decimal point is what determines the number of significant figures in the final answer