Tumbling markets and economic uncertainty in China pose a risk to major chipmakers such as Qualcomm Inc (QCOM.O) that derive a big portion of their sales from the world’s second-largest economy. Consumer electronics giant Apple Inc (AAPL.O) could also be vulnerable – 17 percent of the company’s overall revenue last fiscal year came from China, and in the most recent quarter it sold more iPhones in the country than in the United States for the first time.