19. Select two substances with a molecular structure and a polar covalent bond. 1) CH_(3)COOH 2) KNO_(3) 3) C_(3)H_(8) 4) I_(2) 5) MgO Answer (score: 1):

The correct answer is 1) $CH_{3}COOH$ and 3) $C_{3}H_{8}$.<br /><br />$CH_{3}COOH$ (acetic acid) has a polar covalent bond between the carbon and oxygen atoms in the carboxyl group (-COOH). The oxygen atom is more electronegative than the carbon atom, resulting in a polar covalent bond.<br /><br />$C_{3}H_{8}$ (propane) has polar covalent bonds between the carbon and hydrogen atoms. The carbon atoms are more electronegative than the hydrogen atoms, resulting in polar covalent bonds.<br /><br />The other substances listed do not have polar covalent bonds:<br />- $KNO_{3}$ (potassium nitrate) has ionic bonds between the potassium and nitrate ions.<br />- $I_{2}$ (iodine) has a nonpolar covalent bond between the two iodine atoms.<br />- $MgO$ (magnesium oxide) has an ionic bond between the magnesium and oxygen ions.