91Ó°ÊÓ

Sodium thiosulfate pentahydrate, written as \( \text{Na}_2 \text{S}_2 \text{O}_3 \cdot 5 \text{H}_2 \text{O} \), is an important chemical compound often used in various applications, including photography and water treatment. It's a hydrated salt, meaning it contains water molecules within its crystalline structure. The term "pentahydrate" indicates that there are five water molecules attached to each formula unit of sodium thiosulfate.
This hydration makes sodium thiosulfate pentahydrate distinct from its anhydrous (water-free) form, as the presence of water molecules can notably affect the compound's physical properties like crystal structure and solubility in water.

Density is a key concept when working with solutions, as it helps convert between volume and mass. Density is defined as mass divided by volume, typically expressed in grams per milliliter (g/mL). In the exercise, we're given that the density of the solution is 1.174 g/mL at 20°C. This means 1 milliliter of the solution has a mass of 1.174 grams. Knowing the density is crucial for determining the solution concentration, which refers to how much solute is present in a given volume of solution.
To find concentration by molarity, you often start with the known mass percent and the density to calculate how much solute is present in a particular volume, then convert this into moles using the solute's molar mass.

Molar mass is the mass of a substance divided by the amount of substance (moles) and is expressed in grams per mole (g/mol). For sodium thiosulfate pentahydrate, calculating its molar mass involves adding up the atomic masses of all the atoms in its formula.

• The sodium (Na) content is calculated as \(2 \times 23 = 46\) g/mol.
• The sulfur (S) content is \(2 \times 32 = 64\) g/mol.
• The oxygen (O) content is \(3 \times 16 = 48\) g/mol.

Adding these gives the molar mass for \(\text{Na}_2\text{S}_2\text{O}_3\), totaling 158 g/mol.
Additionally, for the five water molecules, \( 5 \times (2 \times 1 + 16) = 90 \) g/mol is added. Therefore, the total molar mass of sodium thiosulfate pentahydrate is 248 g/mol, which is crucial for further moles-per-volume calculations.

Mass percentage is a common way of expressing concentration in a solution. It indicates the ratio of the mass of the solute to the total mass of the solution, multiplied by 100 to convert it into a percentage. In this case, a 20% mass percentage means there are 20 grams of sodium thiosulfate pentahydrate for every 100 grams of solution. Understanding mass percentage allows you to determine how much of the solute is contained in any amount of solution you are working with.
This percentage can then be used along with the solution's density to figure out the actual concentration, as mass gives a starting point for converting to moles, necessary for finding molarity.

An aqueous solution is simply a solution in which water is the solvent. This means the solute is dissolved in water. Aqueous solutions are central to chemistry because many reactions take place in this medium. In the problem, sodium thiosulfate pentahydrate is dissolved in water which forms the aqueous solution.
Understanding aqueous solutions is important because:

• They allow for chemical reactions to occur in a liquid state.
• Water, being a universal solvent, supports a wide range of solutes, including ionic and molecular compounds.
• They are also essential in understanding how various compounds interact at a molecular level.

Hence, calculating molarity in such solutions becomes a fundamental part of evaluating the concentration and understanding the solution's overall properties.