Transferring and measuring variable volumes requires a device that allows controlled release and precise measurement of how much liquid has been added. A buret is designed for this: it is a long, graduated tube with a stopcock that lets you add liquid drop by drop and read the exact amount delivered by noting the difference between the initial and final volumes. This makes it ideal for experiments like titrations where you need to measure variable, precisely controlled volumes. Other glassware isn’t suited for this purpose. A beaker is used for rough pouring and approximate volumes, not precise delivery. A volumetric pipet delivers a single, fixed volume with high accuracy, not variable amounts. An Erlenmeyer flask is mainly for mixing or heating and has only approximate volume markings.

The precision of a buret reading comes from estimating the volume to one more decimal place than the smallest mark on the instrument. A typical buret has marks every 0.1 mL, but you read the bottom of the meniscus and estimate to the nearest 0.01 mL. This allows you to report measurements with two digits after the decimal, reflecting a hundredth-of-a-milliliter precision. So the most precise reading you can typically claim from a buret is 0.01 mL. Readings that only go to 0.1 mL or 0.5 mL are coarser and don’t take advantage of the interpolated estimation between marks.

The key idea here is to determine an unknown concentration in a sample where the signal depends on the sample’s matrix by using standard additions. You take a fixed amount of the unknown sample and add known amounts of a standard solution containing the same analyte. For each addition, you measure the instrument response. Then you plot the response versus how much standard you added. If the response is linear, the resulting line will cross the x-axis at a value that corresponds to the negative of the unknown concentration in the sample. The magnitude of that intercept gives the unknown concentration in the original sample. This approach works well because each measurement is made in the same matrix, so any matrix effects are carried through and cancel out when you extrapolate to zero added standard. So the described method—adding known volumes of a standard to the unknown, measuring the response, plotting response against added standard, and extrapolating to zero added to find the initial concentration—is the standard addition approach. The other ideas don’t isolate the unknown concentration in the same way: calibrating with the unknown alone doesn’t correct for matrix effects, diluting to match a standard changes the signal without revealing the original amount, and adding the unknown to a standard (or extrapolating to infinite addition) doesn’t provide the same intercept-based determination.

In combustion analysis, the goal is to determine the empirical formula by finding the amounts of carbon, hydrogen, and oxygen in the original sample. The key idea is that burning the sample converts C to CO2 and H to H2O, so measuring those products lets you back-calculate the amounts of C and H. Converting the masses of CO2 and H2O to moles gives you the mole amounts of C and H directly (one mole of CO2 contains one mole of C, and one mole of H2O contains two moles of H). To get oxygen, you balance by difference: subtract the mass of C and H from the original sample mass, and convert that remaining mass to moles of O. Finally, you simplify the three mole values (C, H, O) to the smallest whole-number ratio to obtain the empirical formula. This is exactly the approach described: collect CO2 and H2O, convert to moles of C and H, determine the simplest ratio, and account for oxygen by difference if needed. Using only CO2 or only H2O omits essential information, and assuming all remaining mass is oxygen ignores the presence of other elements, so those alternatives don’t yield the full empirical formula.