The social network’s chief executive officer will owe taxes on gains related to the award of options, the Menlo Park, California-based company said this week in its IPO prospectus. The options were granted to Zuckerberg in 2005 and expire in 2015, and he’ll sell stock to cover liabilities, Facebook said.

“We expect that substantially all of the net proceeds Mr. Zuckerberg will receive upon such sale will be used to satisfy taxes that he will incur upon his exercise of an outstanding stock option to purchase 120,000,000 shares” of common stock, according to the filing.

Zuckerberg, 27, stands to become one of richest people in the world with a stake in Facebook that could be worth as much as $28.4 billion. His company, which filed on Feb. 1 to raise as much as $5 billion in an IPO, is discussing a valuation of $75 billion to $100 billion, two people familiar with the matter said last week.

At the high end of that range, assuming roughly 2.51 billion Facebook shares outstanding, each share may be worth about $39.79. The awarded shares carry an exercise price of 6 cents. Assuming Zuckerberg buys all 120 million shares at that price, his gains would total about $4.77 billion.

Zuckerberg’s tax rate will be 35 percent, said Victor Fleischer, associate professor of law at the University of Colorado. That means his bill would be about $1.67 billion.

Larry Yu, a spokesman for Facebook, declined to comment.

The social network had a valuation of at least $94 billion, or about $40 a share, in an auction of its shares on the private market earlier this week.