Determine the formula of a hydrate
(all examples and problems)


Determine the formula of a hydrate: fifteen examples

Example #1: A 15.67 g sample of a hydrate of magnesium carbonate was heated, without decomposing the carbonate, to drive off the water. The mass was reduced to 7.58 g. What is the formula of the hydrate?

Example #2: A hydrate of Na2CO3 has a mass of 4.31 g before heating. After heating, the mass of the anhydrous compound is found to be 3.22 g. Determine the formula of the hydrate and then write out the name of the hydrate.

Example #3: When you react 3.9267 grams of Na2CO3 · nH2O with excess HCl(aq), 0.6039 grams of a gas is given off. What is the number of water molecules bonded to Na2CO3 (value of n)?

Example #4: If 1.951 g BaCl2 · nH2O yields 1.864 g of anhydrous BaSO4 after treatment with sulfuric acid, calculate n.

Example #5: Given that the molar mass of Na2SO4 · nH2O is 322.1 g/mol, calculate the value of n.

Example #6: 4.92 g of hydrated magnesium sulphate crystals (MgSO4 nH2O) gave 2.40 g of anhydrous magnesium sulfate on heating to a constant mass. Determine the value of n.

Example #7: A 241.3 gram sample of CoCl2 2H2O is heated to dryness. Find the mass of anhydrous salt remaining.

Example #8: A 2.56 g sample of ZnSO4 7H2O is heated to dryness. Determine the anhydrous mass remaining after all the water has been driven off?

Example #9: If 29.0 g of MgSO4 7H2O is thoroughly heated, what mass of anhydrous magnesium sulfate will remain?

Example #10: What is the formula of the hydrate formed when 66.3 g of Ga2(SeO4)3 combines with 33.7 g of H2O?

Example #11: A student determined that the percent of water in a hydrate was 25.3%. The formula of the anhydrous compound was determined to be CuSO4. Calculate the formula of the hydrated compound.

Example #12: 0.572 grams of a hydrate is heated to dryness, ending with 0.498 grams of anhydrous compound. What is the percentage of water by mass in the hydrate?

Example #13: 1.534 grams of BaCl2 · 2H2O is heated to dryness. What will be the mass of BaCl2(s) that remains?

Example #14: 31.0 g of MgSO4 7H2O is thoroughly heated. What mass of anhydrous magnesium sulfate will remain?

Example #15: When a hydrate of Na2CO3 is heated until all the water is removed, it loses 54.3 percent of its mass. Determine the formula of the hydrate.

Bonus Example: 3.20 g of hydrated sodium carbonate, Na2CO3 nH2O was dissolved in water and the resulting solution was titrated against 1.00 mol dm-3 hydrochloric acid. 22.4 cm3 of the acid was required. What is the value of n?


Determine the formula of a hydrate: ten problems

Problem #1: Anhydrous lithium perchlorate (4.78 g) was dissolved in water and re-crystalized. Care was taken to isolate all the lithium perchlorate as its hydrate. The mass of the hydrated salt obtained was 7.21 g. What hydrate is it?

Problem #2: A substance was found to have the following percentages by mass: 23% zinc; 11% sulfur; 22% oxygen; 44% water. What is the empirical formula?

Problem #3: A 5.00 g sample of hydrated barium chloride, BaCl2 · nH2O, is heated to drive off the water. After heating, 4.26 g of anhydrous barium chloride, BaCl2, remains. What is the value of n in the hydrate's formula?

Problem #4: A 1.98 g sample of a cobalt(II) chloride hydrate is heated over a burner. When cooled, the mass of the remaining dehydrated compound is found to be 1.55 g. What is the formula for the original hydrate? How can you make sure that all of the water of hydration has been removed?

Problem #5: A solution was made by dissolving 52.0 g of hydrated sodium carbonate in water and making it up to 5.00 dm3 of solution. The concentration of the solution was determined to be 0.0366 M. Determine the formula of hydrated sodium carbonate.

Problem #6: Determine the formula and name for the hydrate: 73.42% ammonium phosphate and 26.58% water.

Problem #7: 5.00 g of borax (Na2B4O7 · 10H2O) was heated to remove the water. What is the mass of anhydrous sodium tetraborate that remains?

Problem #8: A sample of hydrate lost 14.75% of its original weight during heating. Determine the number of moles of hydration per mole of anhydrous substance if the molecular weight of the anhydrate is 208 grams/mole.

Problem #9: 1.33 g of hydrated ethanedioic acid (H2C2O4 nH2O) were dissolved in distilled water and the solution made up to 250.0 mL in a graduated flask. 25.0 mL of this solution were titrated by 21.1 mL of 0.100M NaOH. Calculate the number of moles of water of crystallization in the hydrated ethanedioic acid.

Problem #10: A solution was made by dissolving 71.5 g of hydrated sodium carbonate in water and making up to 5.00 dm3 of solution. The concentration of a 25.0 cm3 portion was determined to be 0.04805 M. Use the information to find the waters of hydration of the hydrated sodium carbonate.

Bonus Problem: A certain quantity of sodium carbonate decahydrate was heated to remove the water. The mass of the anhydrous compound that remained was 2.764 g. What was the original mass of the hydrated sodium carbonate?


Determine the formula of a hydrate: fifteen problems

Problem #11: 6.9832 g of FeSO4 xH2O is dissolved in water acidified with sulfuric acid. The solution is made up to 250. cm3. 25.00 cm3 of this solution required 25.01 cm3 of 0.0200 M KMnO4 to titrate completely. Calculate x.

Problem #12: A 81.4 gram sample of BaI2 2H2O was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, how many grams of the anhydrous compound remained?

Problem #13: If the hydrated compound UO2(NO3)2 9H2O is heated gently, the water of hydration is lost. If you heat 4.05 g of the hydrated compound to dryness, what mass of UO2(NO3)2 will remain?

Problem #14: Solid copper(II) chloride forms a hydrate of formula CuCl2 xH2O. A student heated a sample of hydrated copper(II) chloride, in order to determine the value of x. The following results were obtained:

mass of crucible = 16.221 g
mass of crucible and hydrated copper(II) chloride = 18.360 g
mass of crucible and anhydrous copper(II) chloride = 17.917 g

From these data, determine the value of x and write the complete formula for hydrated copper(II) chloride.

Problem #15: How many grams of water and anhydrous salt would you get when heating 9.42 g of Fe(NO3)3 9H2O?

Problem #16: Heating 0.695 g CuSO4 nH2O gives a residue of 0.445 g. Determine the value of n.

Problem #17: Epsom salt is MgSO4 nH2O. The hydrate was found to contain 71.4% oxygen. Calculate the number of water molecules associated with each formula unit of magnesium sulfate hydrate.

Problem #18: A sample of hydrated magnesium sulphate, MgSO4 · nH2O, is found to contain 51.1% water. What is the value of n?

Problem #19: If a 9.15 g sample of a hydrated salt produced 6.50 g of anhydrous salt (309.650 g/mol) and 2.65 g of water (18.015 g/mol), what is the molecular mass of the hydrated salt?

Problem #20: A hydrated compound has the formula MCl2 · 2H2O. In an experiment, the following data were determined:

mass of the hydrate: 1.000 g
mass of H2O: 0.185 g

Determine the identity of element M from these results.

Problem #21: A 0.256 g sample of CoCl2 yH2O was dissolved in water, and excess silver salt was added. The silver chloride was filtered, dried, and weighed, and it had a mass of 0.308 g. What is the value of y?

Problem #22: A sample of 0.416 g of CoCl2 yH2O was dissolved in water, and an excess of sodium hydroxide (NaOH) was added. The cobalt hydroxide salt was filtered and heated in a flame, forming cobalt(III) oxide (Co2O3). The mass of cobalt(III) oxide formed was 0.145 g. What is the value of y?

Problem #23: When 5 g of iron(III) chloride hydrate are heated, 2 g of water are driven off . Find the chemical formula of the hydrate.

Problem #24: Cupric chloride, CuCl2, dehydrates when heated. If 0.235 g of CuCl2 xH2O gives 0.185 g of CuCl2 on heating, what is the value of x?

Problem #25: An hydrate of copper (II) chloride has the formula CuCl2 xH2O. The water in a 3.41 g sample of the hydrate was driven off by heating. The remaining sample had a mass of 2.69 g. Find the number of waters of hydration (x) in the hydrate.