Solutions of accurately known concentrations are called

Options

A. standard solutions
B. normal solutions
C. molar solution
D. perfect solutions

Answer

1

Explanation

Often we prepare solutions of substances and then determine their concentrations by titration with a standard solution.
Standardization is the process by which one determines the concentration of a solution by measuring accurately the volume of the solution required to react with an exactly known amount of a primary standard. The standardized solution is then known as a secondary standard and is used in the analysis of unknowns.
The properties of an ideal primary standard include the following:
1. It must not react with or absorb the components of the atmosphere, such as `CO_(2)(g), O_(2)(g) and H_(2)O` vapour.
2. It must react according to one invariable reaction
3. It must have a high percentage purity
4. It should have formula weight to minimize the effect of error in weighing.
5. It must be soluble in the solvent of interest
6. It should be nontoxic
7. It should be readily available (inexpensive).
8. It should be environmentally friendly
The first five of these characteristics are essential to minize the errors involved in analytical methods. The last three characteristics are just as important as the first five in most analytical labories. Because primary standards are often costly and difficult to prepare, secondary standards are often used in day-to-day work.
One primary standard for solutions of acids is sodium carbonate `(Na_(2)CO_(3))`, a solid compound. Sodium carbonate is basically a salt but in the following reaction
`H_(2)SO_(4)+Na_(2)CO_(3)rarrNa_(2)SO_(4)+CO_(2)+H_(2)O`
`Na_(2)CO_(3)` can be thought of as a base, because a base can be broadly defined as a substance that reacts with hydrogen ions.
Most inorgainc bases are metal hydroxides, all of which are solid. However, even in the solid state, most inorganic bases react rapidly with `CO_(2)` (an acid anhydride) from the atomosphere. Most metal hydroxides also absorb `H_(2)O` form the air. These properties make it very difficult to accurately weight out samples of pure metal hydroxides.
Chemists obtain solutions of bases of accurately known concentration by standardizing the solution against an acidic salt, potassium hydrogen phthalate:

This is produced by netralization of the two ionizable hydrogens of an organic acid, phthalic acid `(C_(6)H_(4))_(2)(COOH)_(2)`.
This acid salt, known simply as KHP, has one acidic hydrogen that reacts with bases. `KHP` is easily obtained in a high state of purity and and is soluble in water. It is used for a primary standard for bases.
Note that "ph" in phthalate is silent. Thus, phthalate is pronounced as "thalate". Also not that P in KHP stands for phthlate ion, `C_(6)H_(4)(COO)_(2)^(2-)`, not phosphorus.
Impure samples of acids can be titrated with standard solutions of bases. The results can be used to determine percentage purity of the samples.