SAN FRANCISCO (Reuters) - Google Inc (NasdaqGS:GOOG - News) on Thursday posted a 31 percent rise in net profit on 42 percent revenue growth, defying predictions the company is having trouble converting Web searchers into advertising viewers.

Google shares shot up 11.5 percent to $501 in extended trade.

The Internet leader, one of the hottest technology stocks of 2007, had erased last year's 50 percent gain this year on investor fears that the online advertising industry is maturing and vulnerable to a weakening economy.

First-quarter net income rose to $1.31 billion, or $4.12 per diluted share, from $1.0 billion, or $3.18 cents a share, in the year-earlier quarter. Excluding one-time items and stock option expenses, profit was $4.84 a share.

Gross revenue rose 42 percent to $5.19 billion. By contrast, its revenue grew at a 63 percent rate in the same quarter a year ago.

Wall Street was looking for an average profit, excluding one-items, of $4.53 per share, according to Reuters Estimates. The average forecast for net profit was $3.93 per share.

Revenue was expected, on average, to grow 40 percent to $5.13 billion, according to Reuters Estimates.

(Reporting by Eric Auchard; Editing by Braden Reddall)