NEW YORK (AP) ¿ Standard & Poors said on Tuesday it will add chemicals maker FMC Corp. to its S&P 500 index, replacing Centex Corp. Centex, a Dallas-based homebuilder, is being acquired by Pulte Homes Inc., and shareholders are expected to vote on the deal Aug. 18. S&P said the index change would happen after the close of trading that day. Philadelphia-based FMC had been in the S&P MidCap 400 index. It will be replaced by Informatica Corp., a Redwood City, Calif.-based software maker. Shares of companies added to the S&P 500 often rise, since many investment portfolios are linked to the index. FMC shares rose $1.75, or 3.5 percent, to $51.50 in late trading after Standard & Poors made the announcement. Informatica shares rose 16 cents to $19.