Silver chromate solubility

Silver nitrate Brownish red precipitate of silver chromate soluble in dilute nitric acid and in ammonia solution insoluble in acetic acid... 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

Write the equation for the dissolving of silver chromate, AgjCrO, and the solubility product expression. Silver chromate dissolves to give Ag+ and Cr04 2 ions. 

Example 2. Calculate the solubility product of silver chromate, given that its solubility is 2.5 x 10-2gL-1. 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

The molar solubility of silver chromate, Ag2Cr04, is 6.5 X 10 mol-L. Determine the value of Ksp for silver chromate. 

The common ion effect alters the amount of solid that will dissociate in solution. The addition of solid silver chromate to an aqueous solution of potassium chromate will affect the silver chromate s solubility because —... 

B the ion chromate is common to both solutions and will decrease silver chromate s solubility... 

To illustrate your answer, calculate and compare the molar solubility of silver chloride, AgCl (Kgp = 1.8 x 10 ), with silver chromate,... 

Write the expression for the solubility product of silver chromate, Ag2Cr04. 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

Answer First thing to do is look at the possible products here sodium nitrate and silver chromate. If you remember your solubility rules, you will know that silver chromate is the solid here. Therefore, the reaction will look like ... 

Sample The Ks for silver chromate, Ag2Cr04, is 1.12 X 10 12. What is the molar solubility of Ag2Cr04 ... 

These relationships can be used for more than just describing the status of a solid and its constituent ions. Another useful application is to determine if a precipitate will form from two different solutions. For instance, silver nitrate is soluble in water. Potassium chromate is also soluble (as are all potassium salts). If these two solutions are mixed together, two possible products can form potassium nitrate and silver chromate. Potassium nitrate is soluble, but silver chromate is not. But what if very dilute solutions of each were added together Is there a point at which the solutions would be so dilute that no precipitate would form The ion product tells us that the answer is a definite, Yes. In order for a precipitate to form, the value of Q must exceed Ksp. If it does not, no precipitate will form. Let s take a look at a problem that shows this. 

The precipitate must be Ag2Cr04 because all potassium and nitrate salts are soluble. An important thing to notice is the formula of silver chromate. Each chromate will combine with two silver ions. Since both start out with 0.1 mol, silver is a limiting reactant. Only 0.05 mol of Cr042 will be able to precipitate. At the end of the reaction then, there will be ... 

Example 15 Calculate the solubility product of silver chromate, knowing that 1 litre of the saturated solution contains 3-57 x 10-2 g of dissolved material. The relative molecular mass of Ag2Cr04 is 3317, hence the solubility... 

Fractional precipitation The calculation as to which of two sparingly soluble salts will be precipitated under given experimental conditions may be also made with the aid of the solubility product principle. An example of great practical importance is the Mohr method for the estimation of halides. In this process a solution of chloride ions is titrated with a standard solution of silver nitrate, a small quantity of potassium chromate being added to serve as an indicator. Here two sparingly soluble salts may be formed, viz. silver chloride (a white precipitate) and silver chromate (which is red) ... 

Silver nitrate solution brownish-red precipitate of silver chromate, Ag2Cr04, with a solution of a chromate. The precipitate is soluble in dilute nitric acid and in ammonia solution, but is insoluble in acetic acid. Hydrochloric acid converts the precipitate into silver chloride (white). 

A reddish-brown precipitate of silver dichromate, Ag2Cr207, is formed with a concentrated solution of a dichromate this passes, on boiling with water, into the less soluble silver chromate. 

So far we have considered ionic solids dissolved in pure water. We will now see what happens when the water contains an ion in common with the dissolving salt. For example, consider the solubility of solid silver chromate (Ag2Cr04, Ksp = 9.0 X 10-12) in a 0.100 M solution of AgN03. Before any Ag2Cr04 dissolves, the solution contains the major species Ag+, N03, and H20. Since N03 is not found in Ag2Cr04, we can ignore it. The relevant initial concentrations (before any Ag2Cr04 dissolves) are... 

Silver Chromate, Ag2Cr04, is obtained as. a reddish-brown precipitate on mixing solutions of soluble silve r salts with those of chromate or dichroraate of potassi)Hjnf preferp.bly the former. Wh n prepared... 

Silver chromate is almost insoluble in water, glacial acetic acid, and in solutions of potassium chromate, but soluble in those of ammonia, caustic alkalies, nitrates, and in dilute acetic acid. A concentrated solution of ammonium nitrate is a good crystallising medium for silver chromate. With chlorine, above 200° C., silver chloride, chromium trioxide, and oxygen are produced. The solution in ammonia contains the compound Ag2Cr04.4NH3, which forms crystals isomorphous with the corresponding ammoniacal sulphate. ... 

Various theories have been advanced in explanation. Ostwald suggested that supersaturation took place, followed by precipitation, which cleared the immediate neighbourhood of the reactants, and it was therefore necessary for the silver nitrate to diffuse further before supersaturation was again reached. Hatsehek, however, shows that the periodic precipitation takes place in conditions which render supersaturation impossible. Williams and Mackenzie maintain that the silver chromate is precipitated according to the usual rules of the solubility product, and does not behave in any way as a protected colloid but as a crystalloid. More recent w ork suggests that whenever precipitation takes place, the precipitate first passes through the colloidal... 

Because silver chromate is more soluble, the Ksp value (soluble product constant) of silver chromate is not exceeded until the precipitation of Cl is complete. The endpoint can be corrected by using the Mohr method to standardize the silver nitrate solution against pure sodium chloride. 

Thallium dimethyl chromate is prepared as above, using silver chromate, and forms brilliant yellow plates from water, these decom- x>sing at 255 C. without melting. The chromate is very soluble in water, alcohol, or acetone, and dissolves slightly in ether or light petroleum. 

Like silver chromate, all chromium compounds are colored. Although silver chromate is insoluble in water, potassium chromate (K2Cr04, yellow) and potassium dichromate (K2Cr207, orange) are soluble. 

The molar solubility is calculated from the gram solubility and the molar mass of silver chromate (331.73 g mor ) ... 

The solubility of silver chromate (Ag2Cr04) in 500 mL of water at 25°C is 0.0129 g. Calculate its solubility product constant. 

The chromate ion concentration required to initiate formation of silver chromate under this condition can be computed from the solubility constant for silver chromate. 

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