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（3分）小明在查阅资料时看到这样一段话：“二氧化硫能使紫红色KMnO 4 溶液变成无色溶液，反应方程式为：5SO 2 +2KMnO 4 +2H 2 O ==K 2 SO 4 + 2MnSO 4 + 2 ” 。方程式中最后一种物质的化学式印刷不清楚，小明根据上下文了解到该物质是一种常见的酸，请根据已学知识推测其化学式： ，KMnO 4 中Mn元素化合价为 ， 5SO 2 中“5”的含义是 。
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