What is the percent by mass of sodium chloride in a 1.40 M NaCl solution with a density of 1.07 g/mL?

An element X reacts with oxygen to form XO₂ and with chlorine to form XCl₄. XO₂ is a white solid that melts at high temperatures (above 1000 °C). Under usual conditions, XCl₄ is a colorless liquid with a boiling point of 58 °C. (c) By using a sourcebook such as the CRC Handbook of Chemistry and Physics, try to determine the identity of element X.