Question

the name trisodium phosphate is incorrect for the compound na₃po₄ because:

• you cannot use a prefix for the first element in a molecular compound.
• you cannot use a prefix for the first element of an ionic compound.
• this compound should be called trisodium monophosphate.
• the sodium has a negative 3 charge.
• none of the above

Explanation:

To determine the correct reason, we analyze the compound \( \text{Na}_3\text{PO}_4 \). It is an ionic compound (formed from \( \text{Na}^+ \) and \( \text{PO}_4^{3-} \)). For ionic compounds, we use the criss - cross method to determine the formula from the charges, and we do not use prefixes (like "tri - ", "mono - ") for the first element.

• Option 1: \( \text{Na}_3\text{PO}_4 \) is ionic, not molecular, so this is incorrect.
• Option 2: In ionic compounds, we don't use prefixes for the first element. This matches the nature of \( \text{Na}_3\text{PO}_4 \) (ionic, with \( \text{Na}^+ \) and \( \text{PO}_4^{3-} \), named sodium phosphate as prefixes aren't used for the cation in ionic naming).
• Option 3: The correct name is sodium phosphate, not trisodium monophosphate (prefixes aren't used in ionic compound naming for the cation here).
• Option 4: Sodium has a \( + 1 \) charge, not \( - 3 \).
• Option 5: Since option 2 is correct, this is incorrect.

Answer:

B. you cannot use a prefix for the first element of an ionic compound.