Trộn 200 mL dung dịch HCl có \(\mathrm{pH}=2\) với 300 mL dung dịch NaOH có \(\mathrm{pH}=12\). pH của dung dịch thu được sau khi trộn là
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Tính theo tỉ lệ mol. Từ pH của mỗi dung dịch suy ra nồng độ mol của mỗi chất rồi tính số mol.
Lời giải:
Dung dịch HCl có \(\mathrm{pH}=2\) nên \(\mathrm{C}_{\mathrm{MHCl}}=10^{-2} \mathrm{M}\).
Dung dịch NaOH có \(\mathrm{pH}=12\) nên \(\mathrm{pOH}=2\). Vậy \(\mathrm{C}_{\mathrm{M} \mathrm{NaOH}}=10^{-2} \mathrm{M}\).
\(\mathrm{n}_{\mathrm{HCl}}=2.10^{-3} \mathrm{mol}, \mathrm{n}_{\mathrm{NaOH}}=3.10^{-3} \mathrm{mol}\)
Phản ứng trung hòa: \(\mathrm{NaOH}+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}\) nên \(\mathrm{n}_{\mathrm{NaOH}}\) dư \(=1.10^{-3} \mathrm{mol}\)
pH của dung dịch được quyết định bởi NaOH dư.
\(\Rightarrow C_{O H^{-} d u}=\frac{1.10^{-3}}{0,2+0,3}=2.10^{-3} \mathrm{M} \Rightarrow \mathrm{pOH}=2,7\)
\(\Rightarrow \mathrm{pH}=14-2,7=11,3\)
Chọn C.
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