Đáp án:

 a. \(\text{C}_{\text M\ \text{dd C}}=0,48\ \text{M}\)

b. \( \dfrac{V_{\text{dd A}}}{V_{\text{dd B}}}=\dfrac 21\)

Giải thích các bước giải:

 a. Ta có: \(\dfrac{V_A}{V_B}=\dfrac 23\Leftrightarrow V_{A}=\dfrac 23V_{B}\)

\(\text C_{\text M(C)}=\dfrac{n_{\text{dd}\ \text C}}{V_{\text{dd}\ C}}\\\Leftrightarrow \text C_{\text M(C)}=\dfrac{n_{\text{dd A}}+n_{\text dd B}}{\text V_{\text {dd A}}+\text V_{\text{dd B}}} =\dfrac{0,3\cdot V_{\text{dd A}}+0,6\cdot V_{\text{dd B}}}{\text V_{\text {dd A}}+\text V_{\text{dd B}}}=\dfrac{0,3\cdot \dfrac 23V_{B}+0,6\cdot V_{B}}{\dfrac 23\text V_{\text {dd B}}+\text V_{\text{dd B}}}=0,48\ \text{M}\)

Vậy \(\text{C}_{\text M\ \text{dd C}}=0,48\ \text{M}\)

b. Để nồng độ mol của dung dịch C bằng 0,4 M thì

\(\dfrac{n_{\text {dd C}}}{V_{\text{dd C}}}=0,4\Leftrightarrow \dfrac{0,3\cdot V_{\text{dd A}}+0,6\cdot V_{\text{dd B}}}{V_{\text{dd A}}+V_{\text{dd B}}}=0,4\Leftrightarrow 0,3\cdot V_{\text{dd A}}+0,6\cdot V_{\text{dd B}}=0,4V_{\text{dd A}}+0,4V_{\text{dd B}}\Leftrightarrow 0.2V_{\text{dd B}}= 0,1V_{\text{dd A}}\Leftrightarrow V_{\text{dd A}}=2V_{\text{dd B}}\Leftrightarrow \dfrac{V_{\text{dd A}}}{V_{\text{dd B}}}=\dfrac 21\)