When magnesium chloride (MgCl₂) is dissolved in water, it dissociates into its ions: one magnesium ion (Mg²⁺) and two chloride ions (Cl⁻). Let's analyze the statements provided:
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The sum of the mass of the Mg ions and the mass of the Cl ions must be equal to the mass of MgCl₂.
- This is true because when MgCl₂ dissolves in water, the total mass of the dissolved ions is equal to the original mass of the MgCl₂ that was added.
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The sum of the number of moles of the Mg ions and the number of moles of the Cl ions must be equal to the number of moles of MgCl₂.
- This is NOT true. There is 1 mole of Mg²⁺ for every mole of MgCl₂, but there are 2 moles of Cl⁻ for each mole of MgCl₂. Therefore, the sum of the moles will be 1 + 2 = 3 moles for every mole of MgCl₂, not equal to 1.
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The mass of the Mg ions and the mass of the Cl ions must each be equal to the mass of MgCl₂.
- This is NOT true. The mass of the Mg ions will be less than the total mass of MgCl₂, and similarly, the mass of the Cl ions (which are present as 2 moles per mole of MgCl₂) will also not be equal to the mass of MgCl₂ individually.
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The number of moles of the Mg ions and the number of moles of the Cl ions must each be equal to the number of moles of MgCl₂.
- This is NOT true. There is 1 mole of Mg²⁺ for every mole of MgCl₂, but there are 2 moles of Cl⁻ for every mole of MgCl₂.
Out of these options, the only statement that must be true is the first one: The sum of the mass of the Mg ions and the mass of the Cl ions must be equal to the mass of MgCl₂.
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