A 4.36-g sample of an unknown alkali metal hydroxide is dissolved in 100.0 mL of water. An acid–base indicator is added, and the resulting solution is titrated with 2.50 M HCl(aq) solution. The indicator changes color, signaling that the equivalence point has been reached, after 17.0 mL of the hydrochloric acid solution has been added. (b) What is the identity of the alkali metal cation: Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺?

검증된 단계별 안내

Step 1: Calculate the moles of HCl used in the titration. The molarity of a solution is defined as the moles of solute per liter of solution. Therefore, to find the moles of HCl, multiply the volume of the HCl solution (in liters) by its molarity.

Step 2: Determine the stoichiometry of the reaction. In a neutralization reaction between an acid and a base, the stoichiometry is usually 1:1. This means that one mole of acid reacts with one mole of base. Therefore, the moles of the alkali metal hydroxide (MOH) is equal to the moles of HCl.

Step 3: Calculate the molar mass of the alkali metal hydroxide. The molar mass is the mass of the substance divided by the moles of the substance. In this case, divide the mass of the MOH (in grams) by the moles of MOH calculated in step 2.

Step 4: Subtract the molar mass of OH- (hydroxide ion) from the molar mass of MOH calculated in step 3. This will give you the molar mass of the alkali metal cation.

Step 5: Compare the calculated molar mass of the alkali metal cation with the molar masses of the given alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺). The one that matches or is closest to the calculated molar mass is the identity of the alkali metal cation in the unknown alkali metal hydroxide.

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주요 개념

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Titration and Equivalence Point

Titration is a quantitative analytical method used to determine the concentration of a solute in a solution. The equivalence point occurs when the amount of titrant added is stoichiometrically equivalent to the amount of substance in the sample. In this case, the color change of the indicator signals that all the alkali metal hydroxide has reacted with the hydrochloric acid, allowing for the calculation of the hydroxide's molarity.

Molarity and Stoichiometry

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced equations. By knowing the molarity of the HCl solution and the volume used, one can determine the moles of HCl that reacted, which directly relates to the moles of the alkali metal hydroxide present in the sample.

Alkali Metal Hydroxides

Alkali metal hydroxides are strong bases formed from alkali metals (Li, Na, K, Rb, Cs) and hydroxide ions (OH-). They dissociate completely in water, producing hydroxide ions that react with acids. The identity of the alkali metal can be inferred from the mass of the hydroxide and the amount of acid used in the titration, as each metal hydroxide has a distinct molar mass that influences the total mass of the sample.