Discussion Explain how the calculated molarity of NaOH would be affected by each of the following experimental errors during the standardization of a NaOH solution. In each case you must also state if the calculated molarity is higher or lower than the expected molarity. Be sure to make use of the formula for calculating concentration of NaOH in your answer. Question 7 Not yet answered Marked out of 2.00 Air bubbles at the tip of the burette were not removed before start the titration.
Answers
Explanation:
In the neutralization of hydrochloric acid by sodium hydroxide, the mole ratio of acid to base is 1:1.
text{HCl}(aq)+text{NaOH}(aq) rightarrow text{NaCl}(aq)+text{H}_2text{O}(l)
One mole of HCl would be fully neutralized by one mole of NaOH. If instead the hydrochloric acid was reacted with barium hydroxide, the mole ratio would be 2:1.
2text{HCl}(aq)+text{Ba}(text{OH})_2(aq) rightarrow text{BaCl}_2(aq)+2text{H}_2text{O}(l)
Now two moles of HCl would be required to neutralize one mole of Ba(OH) 2 . The mole ratio insures that the number of moles of H + ions supplied by the acid is equal to the number of OH − ions supplied by the base. This must be the case for neutralization to occur. The equivalence point is the point in a neutralization reaction where the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.
In the laboratory, it is useful to have an experiment where the unknown concentration of an acid or a base can be determined. This can be accomplished by performing a controlled neutralization reaction. A titration is an experiment where a volume of a solution of known concentration is added to a volume of another solution in order to determine its concentration. Many titrations are acid-base neutralization reactions, though other types of titrations can also be performed.
In order to perform an acid-base titration, the chemist must have a way to visually detect that the neutralization reaction has occurred. An indicator is a substance that has a distinctly different color when in an acidic or basic solution. A commonly used indicator for strong acid-strong base titrations is phenolphthalein. Solutions in which a few drops of phenolphthalein have been added turn from colorless to brilliant pink as the solution turns from acidic to basic. The steps in a titration reaction are outlined below.
A measured volume of an acid of unknown concentration is added to an Erlenmeyer flask.
Several drops of an indicator are added to the acid and mixed by swirling the flask.
A buret is filled with the base solution of known molarity.
The stopcock of the buret is opened and base is slowly added to the acid while the flask is constantly swirled to insure mixing. The stopcock is closed at the exact point at which the indicator just changes color.
Phenolphthalein turns pink in basic solutions
Figure 1. Phenolphthalein in basic solution.
The standard solution is the solution in a titration whose concentration is known. In the titration described above the base solution is the standard solution. It is very important in a titration to add the solution from the buret slowly so that the point at which the indicator changes color can be found accurately.
The end point of a titration is the point at which the indicator changes color. When phenolphthalein is the indicator, the end point will be signified by a faint pink color.