Question

consider the substances below.

• 1 mol of beryllium
• 1 mol of salt
• 1 mol of water
• 1 mol of hydrogen

which statement is true about these substances?

• they have exactly the same mass.
• they have different numbers of particles.
• they have the same number of atoms.
• they have different masses.

Explanation:

Step1: Recall Avogadro's Number

Avogadro's number ($N_A = 6.022\times10^{23}$) states that 1 mole of any substance contains $6.022\times10^{23}$ particles (atoms, molecules, formula units, etc.). So, 1 mol of each substance here has the same number of particles. But we need to check mass and number of atoms.

Step2: Analyze Mass

Mass of 1 mole of a substance is its molar mass (in g/mol), which is equal to its atomic/molecular/formula mass (in amu).

• Beryllium (Be) has molar mass ~9 g/mol.
• Salt (NaCl) has molar mass ~58.5 g/mol.
• Water ($H_2O$) has molar mass ~18 g/mol.
• Hydrogen ($H_2$) has molar mass ~2 g/mol.

So, their molar masses are different, so 1 mol of each has different masses.

Step3: Analyze Number of Atoms

• Beryllium: 1 mol of Be has $6.022\times10^{23}$ atoms (1 atom per particle).
• Salt (NaCl): 1 mol of NaCl has $6.022\times10^{23}$ formula units, each with 2 atoms (Na + Cl), so total atoms = $2\times6.022\times10^{23}$.
• Water ($H_2O$): 1 mol of $H_2O$ has $6.022\times10^{23}$ molecules, each with 3 atoms (2 H + 1 O), so total atoms = $3\times6.022\times10^{23}$.
• Hydrogen ($H_2$): 1 mol of $H_2$ has $6.022\times10^{23}$ molecules, each with 2 atoms, so total atoms = $2\times6.022\times10^{23}$.

So, number of atoms is different.

Step4: Evaluate Options

• "They have exactly the same mass.": False, as molar masses differ.
• "They have different numbers of particles.": False, 1 mol of each has $N_A$ particles.
• "They have the same number of atoms.": False, as shown above.
• "They have different masses.": True, since molar masses (and thus 1 mol mass) differ.

Answer:

They have different masses. (The correct option is the one stating "They have different masses.")