题目

The number of speakers of English in Shakespeare’s time is estimated(估计) to have been about five million. Today it is estimated that some 260 million people speak it as a native language, mainly in the United States, Canada, Great Britain, Ireland, South Africa, Australia and New Zealand. In addition to the standard varieties of English found in these areas, there are a great many regional and social varieties of the language as well as various levels of usage that are employed both in its spoken and written forms. In fact, it is impossible to estimate the number of people in the 4．高纯碳酸锰在电子工业中有重要的应用，湿法浸出软锰矿（主要成分为MnO2，含少量Fe、Al、Mg等杂质元素）制备高纯碳酸锰的实验过程如下：（1）浸出：浸出时温度控制在90℃～95℃之间，并且要连续搅拌3小时的目的是提高软锰矿中锰的浸出率，植物粉的作用是作还原剂．（2）除杂：①向浸出液中加入一定量的碳酸锰矿，调节浸出液的pH为3.5～5.5；②再加入一定量的软锰矿和双氧水，过滤；③…操作①中使用碳酸锰调pH的优势是；操作②中加入双氧水不仅能将Fe2+氧化为Fe3+，而且能提高软锰矿的浸出率．写出双氧水提高软锰矿浸出率的离子方程式MnO2+H2O2+2H+=Mn2++2H2O+O2↑．（3）制备：在30℃～35℃下，将碳酸氢铵溶液滴加到硫酸锰净化液中，控制反应液的最终pH在6.5～7.0，得到MnCO3沉淀．温度控制35℃以下的原因是减少碳酸氢铵的分解，提高原料利用率；该反应的化学方程式为MnSO4+2NH4HCO3 $\frac{\underline{\;30-35℃\;}}{\;}$MnCO3+（NH4）2SO4+CO2↑+H2O；生成的MnCO3沉淀需经充分洗涤，检验洗涤是否完全的方法是取最后一次的洗涤滤液1～2mL于试管中，向其中滴加用盐酸酸化的BaCl2溶液，若无白色沉淀产生，则表明已洗涤干净．（4）计算：室温下，Ksp（MnCO3）=1.8×10-11，Ksp（MgCO3）=2.6×10-5，已知离子浓度小于1.0×10-5mol•L-1时，表示该离子沉淀完全．若净化液中的c（Mg2+）=10-2mol/L，试计算说明Mg2+的存在是否会影响MnCO3的纯度．