题目

Some time ago I discovered that one of my chairs had a broken leg. I didn't think there would be any difficulty in getting it mended, as there are a whole lot of antique(古董) shops near my home. So I left home one morning carrying the chair with me. I went into the first shop expecting a friendly reception (接待). I was quite wrong. The man wouldn't even look at my chair. The second shop, though slightly more polite, was just the same, and the third and the fourth－so I decided that my approach must be wrong. I entered the fifth shop with a plan in my mind. I placed the chair on为测定钢铁中硫的含量，称取0.5000g试样，高温下在氧气流中燃烧，使试样中的硫转化为SO2。将燃烧后的气体通入1%的淀粉溶液中进行吸收，并用标准碘溶液进行滴定。 （1）滴定过程中反应的化学方程式是______________________________________，达终点时溶液呈________色； （2）若达滴定终点时，消耗VmL物质的量浓度为a mol/L的碘溶液，则试样中硫的质量分数的计算式为________； （3）为了快速得出测定结果，配制标准碘溶液时，要使每消耗0.1mL碘溶液相当于试样中含硫0.001%，则标准碘溶液的物质的量浓度应是________。