1. Let's first state the problem: We want to calculate the mass of lithium using the atomic mass unit (amu) value. 2. The question is whether to use 13 amu or 15 amu for lithium's atomic mass in the calculation. 3. The atomic mass of lithium is approximately 6.94 amu, but sometimes isotopes or specific contexts might use different values. 4. If the problem or context specifies 13 amu or 15 amu, it likely refers to a different isotope or a different measurement. 5. To calculate the mass of lithium accurately, you should use the correct atomic mass value relevant to the isotope or context. 6. Using 13 amu instead of 15 amu will change the calculated mass proportionally, so ensure you use the value given or the standard atomic mass of lithium. 7. In summary, use the atomic mass value that matches the isotope or problem context; if 13 amu is specified for lithium in your problem, use 13 amu, otherwise use the standard atomic mass.