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Ⅰ.某化学实验小组探究市售食用白醋中醋酸的准确浓度，取25.00mL某品牌食用白醋于烧杯中，稀释并转移至250mL容量瓶中，，摇匀。取稀释液25.00mL用浓度为c b mol/L的标准NaOH溶液对其进行滴定。 （1）该小组用酚酞试液作指示剂，判断到达滴定终点的实验现象是： ____。 （2）右图表示50mL滴定管中液面的位置，若A与C刻度间相差l mL，A处的刻度为25，滴定管中液面读数应为 mL。 （3）为了减小实验误差，该同学一共进行了三次实验，假设每次所取白醋体积均为25.00mL，NaOH标准液浓度为c b mo1/L，三次实验结果记录如下： 实验次数 第一次 第二次 第三次 消耗NaOH溶液体积/mL 26.32 25.34 25.30 从上表可以看出，第一次实验中记录消耗NaOH溶液的体积明显多于后两次，其原因可能是 。 A．滴定前滴定管尖嘴有气泡，滴定结束无气泡 B．盛装标准液的滴定管装液前用蒸馏水洗后，未用标准液润洗 C．第一次滴定用的锥形瓶用白醋润洗 D．该NaOH标准液保存时间过长，有部分变质，浓 度降低 （4）根据所给数据，写出计算该白醋中醋酸的物质的量浓度的表达式(不必化简)： c＝ 。 Ⅱ. 用中和滴定法测定NaOH和Na 2 CO 3 混合溶液中的NaOH的质量分数时，可先在混合溶液中加入过量的BaCl 2 溶液，使Na 2 CO 3 完全变成BaCO 3 沉淀，然后用标准盐酸滴定（用酚酞做为指示剂）。试回答： （1）有BaCO 3 沉淀的NaOH溶液里滴加盐酸，为什么不会使BaCO 3 溶解而能测定NaOH溶液的质量分数？ （2）分析在滴定过程中不断振荡锥形瓶的原因？ （3）滴定终点时溶液颜色如何变化？ （4）能否改用甲基橙作指示剂？并说明理由。
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