题目

补全对话A：I need some school things．   B：I'm sorry，I have to do my homework．Could you get some for me?A：    What do you want?B：Please buy a ruler for me．A：Sure．   B：Yes．I'd like some pencils，too．A：How many do you want?B：   A：A ruler and four pencils．   B：Yes．Thanks．A：You're welcome．A．Is that all?B．Let's go to the shop，shall we?C．Anything else?D．No problem．E．Can I help you?F．Four, please．实验室现有碳酸钠和氯化钠的固体混合物样品．小炜同学想测定样品中碳酸钠的质量分数，先称取该混合物样品20g，完全溶于水中，然后取用了一定溶质质量分数的氯化钙溶液80g平均分4次加入其中，充分振荡，实验所得数据见下表，请你分析并进行有关计算：?实验次数1234加入氯化钙溶液的质量/g20202020生成沉淀的质量/g1.51.5m0.5（1）上表中m数值为______g；?（2）计算原固体混合物样品中碳酸钠的质量分数是多少？