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Chapter 4: Problem 135

The unknown acid \(\mathrm{H}_{2} \mathrm{X}\) can be neutralized completely by \(\mathrm{OH}^{-}\) according to the following (unbalanced) equation: $$ \mathrm{H}_{2} \mathrm{X}(a q)+\mathrm{OH}^{-} \longrightarrow \mathrm{X}^{2-}+\mathrm{H}_{2} \mathrm{O} $$ The ion formed as a product, \(\mathrm{X}^{2-}\), was shown to have 36 total electrons. What is element \(\mathrm{X}\) ? Propose a name for \(\mathrm{H}_{2} \mathrm{X}\). To completely neutralize a sample of \(\mathrm{H}_{2} \mathrm{X}, 35.6 \mathrm{~mL}\) of \(0.175 \mathrm{M} \mathrm{OH}^{-}\) solution was required. What was the mass of the \(\mathrm{H}_{2} \mathrm{X}\) sample used?

Short Answer

Expert verified
The element X is Selenium (Se), and the compound H₂X is called hydrogen selenide or selenous acid. The mass of the H₂X sample used for neutralization is 0.252 g.

Step by step solution

01

Identify Element X

Since the element X forms an ion X²⁻ with 36 electrons, we can find the number of protons (atomic number) in the neutral atom of X. The ion has 2 extra negative charges (electrons), so the neutral atom has 36 - 2 = 34 electrons, which equals the number of protons. Element X is Selenium (Se), which has an atomic number of 34.
02

Provide a Name for H₂X

With the element X identified as Selenium, we can now name the unknown acid H₂X. Since it is an acid with the Se²⁻ ion, it is called hydrogen selenide or selenous acid.
03

Calculate Moles of OH⁻

To find the moles of OH⁻ needed for neutralization, we can use the volume and concentration of the OH⁻ solution: moles of OH⁻ = volume × concentration moles of OH⁻ = 35.6 mL × 0.175 mol/L = 6.23 × 10⁻³ mol
04

Calculate Moles of H₂X

Using the balanced neutralization equation: H₂X + 2 OH⁻ → X²⁻ + 2 H₂O The mole ratio between H₂X and OH⁻ is 1:2, so we can find the moles of H₂X as follows: moles of H₂X = 1/2 × moles of OH⁻ = 1/2 × 6.23 × 10⁻³ mol = 3.115 × 10⁻³ mol
05

Determine the Mass of H₂X Sample

To find the mass of the H₂X sample, we first need to calculate the molar mass of H₂X (H₂Se): M(H₂Se) = 2 × M(H) + M(Se) = 2 × 1.008 g/mol + 78.96 g/mol = 80.976 g/mol Now, we can calculate the mass of the H₂X sample as follows: mass of H₂X = moles of H₂X × molar mass of H₂Se mass of H₂X = 3.115 × 10⁻³ mol × 80.976 g/mol = 0.252 g The mass of the H₂X sample used is 0.252 g.

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