Question

A quantity of \(5 \mathrm{~g}\) of a crystalline salt when rendered anhydrous lost \(1.8 \mathrm{~g}\) of water. The formula mass of the anhydrous salt is 160 . The number of molecules of water of crystallization in the salt is (a) 3 (b) 5 (c) 2 (d) 1

Short answer

The number of molecules of water of crystallization in the salt is 5 (b).

Step-by-step solution

Determine the mass of the anhydrous salt

To find the mass of the anhydrous salt, subtract the mass of water lost from the original mass of the crystalline salt: 5 g - 1.8 g = 3.2 g.

Calculate the number of moles of the anhydrous salt

Using the formula mass of the anhydrous salt (160 g/mol), calculate the number of moles: number of moles = mass (g) / molar mass (g/mol), which gives, number of moles = 3.2 g / 160 g/mol = 0.02 moles.

Calculate the mass of water in moles

The mass of water lost is 1.8 g. Since the molar mass of water (H2O) is approximately 18 g/mol, the number of moles of water is 1.8 g / 18 g/mol = 0.1 moles.

Determine the ratio of moles of water to moles of salt

The number of water molecules per formula unit of the salt is the ratio of moles of water to moles of salt, which is 0.1 moles / 0.02 moles = 5.

Identify the number of molecules of water of crystallization

Since the ratio of moles of water to moles of salt is 5:1, this indicates there are 5 molecules of water of crystallization in the salt.

Key concepts

Understanding Molar Mass

The molar mass is a fundamental concept in chemistry, representing the mass of one mole of a given substance. It is expressed in grams per mole (g/mol) and can be calculated using the relative atomic masses of the elements as listed in the periodic table. To put it simply, if you think of a mole as a 'chemical dozen,' the molar�m�m�ass tells us the weight of one of those 'dozens.' For instance, the molar mass of water (H₂O) is approximately 18 g/mol since it is composed of two hydrogen atoms (1 g/mol each) and one oxygen atom (16 g/mol).

Moles Calculation Explained

The concept of moles is essential for quantifying substances in chemistry. A mole represents Avogadro's number of units (um�6.022�14076�1023�mol1�), which can be atoms, molecules, ions, or electrons. To calculate the number of moles, you need to divide the given mass of a substance by its molar mass. This step is crucial in stoichiometric calculations, as it allows chemists to use a consistent method to measure atoms and molecules in a given sample.

For example, the exercise demonstrates calculating the moles of anhydrous salt by dividing its mass (3.2 g) by its molar mass (160 g/mol), yielding 0.02 moles.

Stoichiometry: The Art of Measuring in Chemistry

Stoichiometry is the section of chemistry that deals with the measurement of the quantities of reactants and products in chemical reactions. It's based on the laws of conservation of mass and the concept of moles, enabling chemists to predict the outcomes of chemical reactions. When conducting stoichiometric calculations, you often use balanced chemical equations to find out the stoichiometric coefficients, which indicate the ratio in which elements and compounds react and are formed. In everyday terms, it's like using a detailed recipe to bake a perfect cake, ensuring that the quantities of ingredients are proportional.

The textbook exercise presents a stoichiometric problem where the number of molecules of water of crystallization in a salt is determined by finding the ratio of moles of water to moles of the anhydrous salt.

Crystalline Salt Hydration

Crystalline salts often contain water of crystallization, which refers to water molecules that are integrally bound within the crystal structure of the salt. These water molecules play a key role in the physical properties of the crystalline material, such�as its shape, color, and stability. The water of crystallization is not loosely held; it's a part of the chemical formula of the salt. When a salt has a specific number of water molecules associated with it, these are referred to in terms such as 'hydrate' and numerically indicated in the formula (for example, CuSO₄�5H₂O).

Understanding crystalline salt hydration is essential when analyzing the composition of hydrates, as shown in our example where the crystalline salt contained five molecules of water of crystallization for each formula unit of the salt.